U.S. patent application number 11/075376 was filed with the patent office on 2006-04-20 for leadless cardiac stimulation systems.
Invention is credited to Roger N. Hastings, Graig Kveen, Michael J. Pikus, Anupama Sadasiva.
Application Number | 20060085042 11/075376 |
Document ID | / |
Family ID | 35788751 |
Filed Date | 2006-04-20 |
United States Patent
Application |
20060085042 |
Kind Code |
A1 |
Hastings; Roger N. ; et
al. |
April 20, 2006 |
Leadless cardiac stimulation systems
Abstract
Various configurations of systems that employ leadless
electrodes to provide pacing therapy are provided. In one example,
a system that provides multiple sites for pacing of myocardium of a
heart includes wireless pacing electrode assemblies that are
implantable at sites proximate the myocardium using a percutaneous,
transluminal, catheter delivery system. Also disclosed are various
configurations of such systems, wireless electrode assemblies, and
delivery catheters for delivering and implanting the electrode
assemblies.
Inventors: |
Hastings; Roger N.; (Maple
Grove, MN) ; Sadasiva; Anupama; (Plymouth, MN)
; Pikus; Michael J.; (Golden Valley, MN) ; Kveen;
Graig; (Maple Grove, MN) |
Correspondence
Address: |
FISH & RICHARDSON P.C.
PO BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Family ID: |
35788751 |
Appl. No.: |
11/075376 |
Filed: |
March 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10971550 |
Oct 20, 2004 |
|
|
|
11075376 |
Mar 7, 2005 |
|
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Current U.S.
Class: |
607/33 |
Current CPC
Class: |
A61N 1/37223 20130101;
A61N 1/3627 20130101; A61N 1/0587 20130101; A61N 1/3756 20130101;
A61N 1/3787 20130101 |
Class at
Publication: |
607/033 |
International
Class: |
A61N 1/362 20060101
A61N001/362 |
Claims
1. A catheter delivery system for implantation of at least a
portion of a wireless electrode assembly through endocardium tissue
and into myocardium tissue, comprising: a first elongate member
having a proximal end and a distal end and defining a lumen passing
therethrough; a second elongate member having a proximal end and a
distal end; and a wireless electrode assembly attachable to the
distal end of the second elongate member, wherein when the
electrode assembly is attached to the second elongate member, the
second elongate member is passable through the lumen to deliver at
least a portion of the electrode assembly through the endocardium
and into the myocardium.
2. The system of claim 1, wherein the electrode assembly includes
an attachment mechanism having at least one fastener to penetrate
through the endocardium and into the myocardium.
3. The system of claim 2, wherein the attachment mechanism is
operable to secure at least a portion of the electrode assembly to
the myocardium.
4. The system of claim 2, wherein the attachment mechanism includes
at least one helical tine and at least one curled tine.
5. The system of claim 4, wherein the attachment mechanism includes
a distally extending helical tine to penetrate through the
endocardium and into the myocardium and a plurality of radially
extending curled tines.
6. The system of claim 2, wherein the fastener of the attachment
mechanism includes a tine, screw, barb, or hook.
7. The system of claim 1, wherein the second elongate member has a
detachment mechanism at the distal end to release the electrode
assembly from the second elongate member after delivery of the
electrode assembly to the myocardium.
8. The system of claim 7, wherein the detachment mechanism includes
a threaded member that releasably engages a portion of the
electrode assembly.
9. The system of claim 7, wherein the detachment mechanism includes
an adjustable locking member that releasably engages a portion of
the electrode assembly.
10. The system of claim 1, wherein the first elongate member
includes a steering mechanism to direct the distal end of the first
elongate member to a selected site proximate to the
endocardium.
11. The system of claim 10, wherein the first elongate member
includes an electrode at its distal end for sensing a local
electrocardiogram at the selected site proximate to the
endocardium.
12. The system of claim 1, further comprising an access catheter
having a proximal end and a distal end and having a lumen passing
therethrough, wherein the first elongate member is a delivery
catheter that is passable through the lumen of the access
catheter.
13. The system of claim 12, further comprising an image device near
the distal end of the access catheter.
14. The system of claim 13, wherein the image device includes an
ultrasonic device to provide visualization of a selected site
distal of the access catheter.
15. An implantable wireless electrode assembly comprising: a first
electrode to discharge a pacing electrical pulse; and an attachment
mechanism having at least one fastener to penetrate through
endocardium tissue and into myocardium tissue, at least a portion
of the attachment mechanism being disposed proximate to the
electrode such that, when the fastener penetrates through the
endocardium and into the myocardium, the electrode is positioned
proximate to the myocardium.
16. The wireless electrode assembly of claim 15, further comprising
a second electrode spaced apart from the first electrode such that,
when the fastener penetrates through endocardium and into the
myocardium, the first electrode is positioned proximate to the
myocardium while the second electrode is exposed to blood in an
internal heart chamber.
17. The wireless electrode assembly of claim 15, further comprising
an induction device to receive electromagnetic energy from an
external source, wherein the first electrode is electrically
connected to a circuit such that the pacing electrical pulse is
generated from at least a portion of the electromagnetic energy
received by the induction device.
18. The wireless electrode assembly of claim 15, wherein the
circuit includes an energy storage element to store the
electromagnetic energy received by the induction device, the energy
storage element being operable to periodically discharge electrical
energy to the electrode.
19. The wireless electrode assembly of claim 15, wherein the
induction device includes a coil that is inductively coupled to the
external source.
20. The wireless electrode assembly of claim 15, wherein the
attachment mechanism includes at least one helical tine and at
least one curled tine.
21. The wireless electrode assembly of claim 20, wherein the
attachment mechanism includes a distally extending helical tine to
penetrate through the endocardium and into the myocardium and
includes a plurality of radially extending tines that are adapted
to a curl into the endocardium or myocardium after the helical tine
penetrates into the myocardium.
22. The wireless electrode assembly of claim 15, wherein the
fastener of the attachment mechanism includes a tine, screw, barb,
or hook.
23. The wireless electrode assembly of claim 15, further comprising
a detachment mechanism spaced apart from the fastener of the
attachment mechanism, the detachment mechanism including a threaded
member and being operable to release the wireless electrode
assembly from a delivery system after the fastener penetrates
through endocardium and into the myocardium.
24. A method of delivering a wireless electrode assembly into an
internal heart chamber and proximate the myocardium, the method
comprising: directing a distal end of a first elongate member into
an internal heart chamber, the first elongate member having the
distal end and a proximal end and having a lumen passing
therethrough; directing a wireless electrode assembly through the
lumen of the first elongate member toward the distal end of the
first elongate member; and penetrating at least a portion of the
wireless electrode assembly through endocardium tissue and into the
myocardium.
25. The method of claim 24, wherein the wireless electrode assembly
is attached to a distal end of a second elongate member that is
passable through the lumen of the first elongate member, the method
further comprising operating a detachment mechanism to release the
wireless electrode assembly from the first elongate member.
26. The method of claim 25, further comprising withdrawing the
second elongate member and the first elongate member away from
endocardium.
27. The method of claim 24, further comprising measuring a local
electrocardiogram with a sensor at the distal end of the first
elongate member after at least a portion of the wireless electrode
assembly penetrates the endocardium.
28. The method of claim 27, further comprising deploying one or
more adjustable tines of the wireless electrode assembly after
measuring the local electrocardiogram.
29. The method of claim 27, further comprising withdrawing the
wireless electrode assembly from the myocardium after measuring the
local electrocardiogram and penetrating at least a portion of the
wireless electrode assembly through a different portion the
endocardium and into a different portion of the myocardium.
30. The method of claim 24, wherein penetrating at least a portion
of the wireless electrode assembly through endocardium tissue and
into the myocardium comprises causing an attachment mechanism of
the electrode assembly to penetrate through the endocardium.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/971,550, filed on Oct. 20, 2004, the entire
content of which is hereby incorporated by reference.
TECHNICAL FIELD
[0002] This document relates to systems that electrically stimulate
cardiac or other tissue and that do so without using leads that
extend into the heart or other surrounding tissue or organs, along
with systems and methods for introducing such stimulators.
BACKGROUND
[0003] Pacemakers provide electrical stimulus to heart tissue to
cause the heart to contract and hence pump blood. Conventionally,
pacemakers include a pulse generator that is implanted, typically
in a patient's pectoral region just under the skin. One or more
leads extend from the pulse generator and into chambers of the
heart, most commonly into the right ventricle and the right atrium,
although sometimes also into a vein over the left chambers of the
heart. An electrode is at a far end of a lead and provides the
electrical contact to the heart tissue for delivery of the
electrical pulses generated by the pulse generator and delivered to
the electrode through the lead.
[0004] The conventional use of leads that extend from the pulse
generator and into the heart chambers has various drawbacks. For
example, leads have at their far ends a mechanism, such as tines or
a "j-hook," that causes the lead to be secured to a tissue region
where a physician positions the lead. Over time, the heart tissue
becomes intertwined with the lead to keep the lead in place.
Although this is advantageous in that it ensures the tissue region
selected by the physician continues to be the region that is paced
even after the patient has left the hospital, it is also
disadvantageous in the event of a lead failure or in the event it
is later found that it would be more desirable to pace a different
location than the tissue region initially selected. Failed leads
cannot always be left in the patient's body, due to any potential
adverse reaction the leads may have on heart function, including
infection, thrombosis, valve dysfunction, etc. Therefore, difficult
lead removal procedures sometimes must be employed.
[0005] The conventional use of leads also limits the number of
sites of heart tissue at which electrical energy may be delivered.
The reason the use of leads is limiting is that leads most commonly
are positioned within cardiac veins. As shown in FIG. 17, up to
three leads 2, 3 and 4 are implanted in conventional pacing systems
that perform multiple-site pacing of the heart 1, with the leads
exiting the right atrium 5 via the superior vena cava 6. Multiple
leads may block a clinically significant fraction of the cross
section of the vena cava and branching veins leading to the
pacemaker implant.
[0006] No commercial pacing lead has been indicated for use in the
chambers of the left side of the heart. This is because the high
pumping pressure on the left side of the heart may eject a thrombus
or clot that forms on a lead or electrode into distal arteries
feeding critical tissues and causing stroke or other embolic
injury. Thus, conventional systems, as shown in FIG. 17, designed
to pace the left side of the heart thread a pacing lead 2 through
the coronary sinus ostium 7, located in the right atrium 5, and
through the coronary venous system 8 to a location 9 in a vein over
the site to be paced on the left side. While a single lead may
occlude a vein over the left heart locally, this is overcome by the
fact that other veins may compensate for the occlusion and deliver
more blood to the heart. Nevertheless, multiple leads positioned in
veins would cause significant occlusion, particularly in veins such
as the coronary sinus that would require multiple side-by-side
leads.
[0007] There are several heart conditions that may benefit from
pacing at multiple sites of heart tissue. One such condition is
congestive heart failure (CHF). It has been found that CHF patients
have benefited from bi-ventricular pacing, that is, pacing of both
the left ventricle and the right ventricle in a timed relationship.
Such therapy has been referred to as "resynchronization therapy."
It is believed that many more patients could benefit if multiple
sites in the left and right ventricles could be synchronously
paced. In addition, pacing at multiple sites may be beneficial
where heart tissue through which electrical energy must propagate
is scarred or dysfunctional, which condition halts or alters the
propagation of an electrical signal through that heart tissue. In
these cases multiple-site pacing may be useful to restart the
propagation of the electrical signal immediately downstream of the
dead or sick tissue area. Synchronized pacing at multiple sites on
the heart may inhibit the onset of fibrillation resulting from slow
or aberrant conduction, thus reducing the need for implanted or
external cardiac defibrillators. Arrhythmias may result from slow
conduction or enlargement of the heart chamber. In these diseases,
a depolarization wave that has taken a long and/or slow path around
a heart chamber may return to its starting point after that tissue
has had time to re-polarize. In this way, a never ending
"race-track" or "circus" wave may exist in one or more chambers
that is not synchronized with normal sinus rhythm. Atrial
fibrillation, a common and life threatening condition, may often be
associated with such conduction abnormalities. Pacing at a
sufficient number of sites in one or more heart chambers, for
example in the atria, may force all tissue to depolarize in a
synchronous manner to prevent the race-track and circus rhythms
that lead to fibrillation.
[0008] Systems using wireless electrodes that are attached to the
epicardial surface of the heart to stimulate heart tissue have been
suggested as a way of overcoming the limitations that leads pose.
In the suggested system, wireless electrodes receive energy for
generating a pacing electrical pulse via inductive coupling of a
coil in the electrode to a radio frequency (RF) antenna attached to
a central pacing controller, which may also be implanted. The
wireless electrodes are screwed into the outside surface of the
heart wall.
SUMMARY
[0009] The invention is directed to various configurations of
systems that employ leadless electrodes to provide pacing therapy
and that are commercially practicable. One of the findings of the
inventors is that a significant issue to be considered in achieving
a commercially practicable system is the overall energy efficiency
of the implanted system. For example, the energy transfer
efficiency of two inductively coupled coils decreases dramatically
as the distance between the coils increases. Thus, for example, a
transmitter coil implanted in the usual upper pectoral region may
only be able to couple negligible energy to a small seed electrode
coil located within the heart.
[0010] One aspect of the invention may include a catheter delivery
system for implantation of at least a portion of a wireless
electrode assembly through endocardium tissue and into myocardium
tissue. The catheter delivery system may include a first elongate
member having a proximal end and a distal end and defining a lumen
passing therethrough. The system may also include a second elongate
member having a proximal end and a distal end. The system may
further include a wireless electrode assembly attachable to the
distal end of the second elongate member. When the electrode
assembly is attached to the second elongate member, the second
elongate member may be passable through the lumen to deliver at
least a portion of the electrode assembly through the endocardium
and into the myocardium.
[0011] In some embodiments, the electrode assembly may include an
attachment mechanism that has at least one fastener to penetrate
through the endocardium and into the myocardium. The attachment
mechanism may be operable to secure at least a portion of the
electrode assembly to the myocardium. In some instances, the
attachment mechanism may include at least one helical tine and at
least one curled tine. For example, the attachment mechanism may
include a distally extending helical tine to penetrate through the
endocardium and into the myocardium and a plurality of radially
extending curled tines. In other instances, the fastener of the
attachment mechanism may include a tine, screw, barb, or hook.
[0012] In further embodiments, the second elongate member may have
a detachment mechanism at the distal end to release the electrode
assembly from the second elongate member after delivery of the
electrode assembly to the myocardium. In some instances, the
detachment mechanism may include a threaded member that releasably
engages a portion of the electrode assembly. In other instances,
the detachment mechanism may include an adjustable locking member
that releasably engages a portion of the electrode assembly.
[0013] In certain embodiments, the first elongate member includes a
steering mechanism to direct the distal end of the first elongate
member to a selected site proximate to the endocardium. The first
elongate member may include an electrode at its distal end for
sensing a local electrocardiogram at the selected site proximate to
the endocardium.
[0014] In some embodiments, the system also includes an access
catheter having a proximal end and a distal end and having a lumen
passing therethrough, The first elongate member may be a delivery
catheter that is passable through the lumen of the access catheter.
An image device may be disposed near the distal end of the access
catheter. The image device may include an ultrasonic device to
provide visualization of a selected site distal of the access
catheter.
[0015] In another aspect, an implantable wireless electrode
assembly may include a first electrode to discharge a pacing
electrical pulse. The assembly may also include an attachment
mechanism having at least one fastener to penetrate through
endocardium tissue and into myocardium tissue. At least a portion
of the attachment mechanism may be disposed proximate to the
electrode such that, when the fastener penetrates through the
endocardium and into the myocardium, the electrode is positioned
proximate to the myocardium.
[0016] In some embodiments, the wireless electrode assembly also
includes a second electrode. The second electrode may be spaced
apart from the first electrode such that, when the fastener
penetrates through endocardium and into the myocardium, the first
electrode is positioned proximate to the myocardium while the
second electrode is exposed to blood in an internal heart
chamber.
[0017] In further embodiments, the wireless electrode assembly may
also include an induction device to receive electromagnetic energy
from an external source. The first electrode may be electrically
connected to a circuit such that the pacing electrical pulse is
generated from at least a portion of the electromagnetic energy
received by the induction device. The circuit may include an energy
storage element to store the electromagnetic energy received by the
induction device. The energy storage element may be operable to
periodically discharge electrical energy to the electrode.
[0018] In certain embodiments, the wireless electrode assembly may
include a induction device comprising a coil that is inductively
coupled to the external source.
[0019] In some embodiments, the wireless electrode assembly may
include an attachment mechanism that comprises at least one helical
tine and at least one curled tine. The attachment mechanism may
include a distally extending helical tine to penetrate through the
endocardium and into the myocardium and may include a plurality of
radially extending tines that are adapted to a curl into the
endocardium or myocardium after the helical tine penetrates into
the myocardium.
[0020] In other embodiments, the wireless electrode assembly may
include attachment mechanism that comprises a tine, screw, barb, or
hook.
[0021] In further embodiments, the wireless electrode assembly also
includes a detachment mechanism spaced apart from the fastener of
the attachment mechanism. The detachment mechanism may include a
threaded member and may be operable to release the wireless
electrode assembly from a delivery system after the fastener
penetrates through endocardium and into the myocardium.
[0022] Yet another aspect may include a method of delivering a
wireless electrode assembly into an internal heart chamber and
proximate the myocardium. The method may include directing a distal
end of a first elongate member into an internal heart chamber. The
first elongate member may have the distal end, a proximal end, and
a lumen passing therethrough. The method may also include directing
a wireless electrode assembly through the lumen of the first
elongate member toward the distal end of the first elongate member.
The method may further include penetrating at least a portion of
the wireless electrode assembly through endocardium tissue and into
the myocardium.
[0023] In some embodiments, the method may employ a wireless
electrode assembly that is attached to a distal end of a second
elongate member. The second elongate member may be passable through
the lumen of the first elongate member. In such cases, the method
may also include operating a detachment mechanism to release the
wireless electrode assembly from the first elongate member.
Furthermore, the method may also include withdrawing the second
elongate member and the first elongate member away from
endocardium.
[0024] In certain embodiments, the method may also include
measuring a local electrocardiogram with a sensor at the distal end
of the first elongate member after at least a portion of the
wireless electrode assembly penetrates the endocardium. In such
cases, the method may also include deploying one or more adjustable
tines of the wireless electrode assembly after measuring the local
electrocardiogram. In certain circumstances, the method may include
withdrawing the wireless electrode assembly from the myocardium
after measuring the local electrocardiogram and penetrating at
least a portion of the wireless electrode assembly through a
different portion the endocardium and into a different portion of
the myocardium.
[0025] In some embodiments, the operation of penetrating at least a
portion of the wireless electrode assembly through endocardium
tissue includes causing an attachment mechanism of the electrode
assembly to penetrate through the endocardium.
[0026] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0027] FIG. 1 is a conceptual diagram of a leadless cardiac
stimulation system (with leadless, or wireless, electrode
assemblies shown implanted in a heart) and of an external
programmer.
[0028] FIGS. 2A and 2B are exemplary systems of the type shown in
FIG. 1, and shown implanted in a body.
[0029] FIG. 3 is a block diagram of an exemplary embodiment of a
combined controller/transmitter device and associated antenna that
may be used as part of the FIGS. 2A or 2B system.
[0030] FIG. 4 is a schematic diagram of a portion of the circuitry
included in a wireless electrode assembly as is shown in FIGS. 1
and 2A-B.
[0031] FIG. 5 is a flow chart of a method of providing stimulation
pulses in a pacing cycle in a system such as shown in FIGS. 1 and
2A-B.
[0032] FIG. 6 is a diagram of the system shown in FIG. 2A and of an
example wireless electrode assembly delivery catheter.
[0033] FIG. 7 is a side-view diagram of the delivery catheter shown
in FIG. 6, with portions removed to show a wireless electrode
assembly and additional assemblies inside the catheter.
[0034] FIG. 8 is a diagram similar to FIG. 7, with a distal end of
the delivery catheter pressed against a myocardial wall.
[0035] FIG. 9 is a diagram illustrating the delivery of a wireless
electrode assembly from the delivery catheter and into the
myocardial wall.
[0036] FIG. 10 is a flow chart of a method for delivering and
implanting wireless electrode assemblies.
[0037] FIGS. 11A-D are diagrams of alternative embodiments of
wireless electrode assemblies and associated delivery catheters,
with the wireless electrode assemblies shown being implanted within
a myocardial wall.
[0038] FIGS. 11E-W are diagrams of alternative embodiments of
wireless electrode assemblies and associated delivery
catheters.
[0039] FIG. 12 is a diagram of a wireless electrode assembly and
associated delivery catheter, with the wireless electrode assembly
shown implanted within a myocardial wall in a position such that
its longitudinal axis is parallel with the myocardial wall.
[0040] FIG. 13 is a diagram of a wireless electrode assembly and an
another embodiment of an associated delivery catheter.
[0041] FIGS. 14A and 14B are diagrams of an alternative embodiment
of a wireless electrode assembly and associated delivery catheter,
with the wireless electrode assembly being shown being implanted
within a myocardial wall.
[0042] FIG. 15 is a diagram of an alternative embodiment of a coil
for a wireless electrode assembly in which three orthogonal coils
are wound on a single substrate.
[0043] FIG. 16 is a part schematic and part block diagram of a
circuit that may be included within embodiments of wireless
electrode assemblies to enable them to receive and to transmit
information.
[0044] FIG. 17 is an example of a prior art, three-lead pacing
system, showing one lead placed in a vein over the left
ventricle.
[0045] FIGS. 18A to 18C show views of a wireless electrode assembly
and a wireless electrode assembly attached to a tissue equivalent
circuit.
[0046] FIG. 19 is a graph of voltage, both computed and measured,
induced in a wireless electrode assembly versus time.
[0047] FIG. 20 is a graph of voltage induced in a particular
wireless electrode assembly versus time, with and without a tissue
equivalent circuit attached across the electrodes.
[0048] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0049] This document describes various configurations of systems
that employ leadless electrodes to provide pacing therapy or other
tissue excitation and that are commercially practicable. One of the
findings of the inventors is that a significant issue to be
considered in achieving a commercially practicable system is the
overall energy efficiency of the implanted system. For example, the
energy transfer efficiency of two inductively coupled coils
decreases dramatically as the distance between the coils increases.
Thus, for example, a transmitter coil implanted in the usual upper
pectoral region may only be able to couple negligible energy to a
small seed electrode coil located within the heart.
[0050] FIG. 1 shows a general depiction of such a system 10 and an
external programming device 70. The system 10 includes a number of
wireless electrode assemblies 20, herein referred to simply as
"seeds." The seeds 20 are implanted within chambers of the heart
30. In this example, there are eight seeds 20, there being one
implanted in the left atrium 32, three implanted in the left
ventricle 34, one implanted in the right atrium 36, and three
implanted in the right ventricle 38. In one embodiment, each of the
seeds 20 has an internal coil that is inductively coupled with an
external power source coil to charge an electrical charge storage
device contained within the seed 20, and also has a triggering
mechanism to deliver stored electrical charge to adjacent heart
tissue.
[0051] In another embodiment, one or more of the seeds has no
energy storage device such as a battery or capacitor. In such a
situation, each seed may be comprised, for example, of a ferrite
core having caps at each end with ring electrodes encircling the
caps, so as to form a dumbbell-shaped configuration. A number of
turns of fine insulated wire may be wrapped around the central
portion of the core so as to receive energy from a magnetic field
produced by a shaped driving signal and designed to activate the
electrodes. Such a configuration is discussed below in greater
detail with reference to FIGS. 18A to 18C.
[0052] Referring again to FIG. 1, the system 10 also includes a
pacing controller 40 and a transmitter 50 that drives an antenna 60
for communication with the seeds 20. Generally, the pacing
controller 40 includes circuitry to sense and analyze the heart's
electrical activity, and to determine if and when a pacing
electrical pulse needs to be delivered and by which of the seeds
20. The sensing capability may be made possible by having sense
electrodes included within the physical assembly of the pacing
controller 40. Alternatively, a conventional single or dual lead
pacemaker (not shown in FIG. 1; although see FIG. 2B) may sense the
local cardiac electrocardiogram (ECG) and transmit this information
to antenna 60 for use by controller 40 in determination of the
timing of seed firing. In either case, the seed 20 need not be
provided with sensing capability, and also the seeds 20 need not be
equipped with the capability of communicating to the pacing
controller 40 (for example, to communicate information about sensed
electrical events). In alternative embodiments, the seeds may
communicate sensed information to each other and/or to the
controller 40.
[0053] The transmitter 50--which is in communication with, and is
controlled by, the pacing controller 40--drives an RF signal onto
the antenna 60. In one embodiment, the transmitter 50 provides both
1) a charging signal to charge the electrical charge storage
devices contained within the seeds 20 by inductive coupling, and 2)
an information signal, such as a pacing trigger signal, that is
communicated to a selected one or more of the seeds 20, commanding
that seed to deliver its stored charge to the adjacent tissue.
[0054] An important parameter of the seed 20 that is a driver of
the system 10 design is the maximum energy required to pace the
ventricle. This energy requirement can include a typical value
needed to pace ventricular myocardium, but also can include a
margin to account for degradation of contact between the electrodes
and tissue over time. It is assumed that each seed may require the
maximum pacing threshold energy. This threshold energy is supplied
to the seeds between heartbeats by an external radio frequency
generator (which may also be implanted), or other suitable energy
source that may be implanted within the body. Typical values are:
[0055] Threshold pacing voltage=2.5 Volts [0056] Typical lead
impedance=600 Ohms [0057] Typical pulse duration=0.4 mSec [0058]
Derived threshold energy=4 micro-Joules Because RF fields at
frequencies higher than about 100 kHz are attenuated by the body's
electrical conductivity, and because electric fields of any
frequency are attenuated within the body, energy transmission
through the body may be accomplished via a magnetic field at about
20-100 kHz (or by a magnetic field pulse that contains major
frequency components in this range), and preferably by transmission
of magnetic fields in the range of 20-30 kHz when transmission is
through relatively conductive blood and heart muscle.
[0059] As will be seen later in some of the specifically described
configurations of the system 10, the pacing controller 40 and the
transmitter 50 may be housed in a single enclosure that is body
implantable within a patient. In such a configuration, the single
enclosure device may have a single energy source (battery) that may
be either rechargeable or non-rechargeable. In another
configuration, the pacing controller 40 and the transmitter 50 may
be physically separate components. As an example of such a
configuration, the pacing controller 50 may be implantable, for
example in the conventional pacemaker configuration, whereas the
transmitter 50 (along with the antenna 60) may be adapted to be
worn externally, such as in a harness that is worn by the patient.
In the latter example, the pacing controller 40 would have its own
energy source (battery), and that energy would not be rechargeable
given the relatively small energy requirements of the pacing
controller 40 as compared to the energy requirements of the
transmitter 50 to be able to electrically charge the seeds 20. In
this case, the pacing controller 40 would sense the local cardiac
ECG signal through a conventional pacing lead, and transmit the
sensed information to the external controller. Again, transmission
of information, as opposed to pacing energy, has a relatively low
power requirement, so a conventional pacemaker enclosure and
battery would suffice.
[0060] The external programmer 70 is used to communicate with the
pacing controller 40, including after the pacing controller 40 has
been implanted. The external programmer 70 may be used to program
such parameters as the timing of stimulation pulses in relation to
certain sensed electrical activity of the heart, the energy level
of stimulation pulses, the duration of stimulation pulse (that is,
pulse width), etc. The programmer 70 includes an antenna 75 to
communicate with the pacing controller 40, using, for example, RF
signals. The implantable pacing controller 40 is accordingly
equipped to communicate with the external programmer 70, using, for
example, RF signals. The antenna 60 may be used to provide such
communications, or alternatively, the pacing controller 40 may have
an additional antenna (not shown in FIG. 1) for external
communications with the programmer 70, and in an embodiment where
the transmitter 50 and antenna 60 are housed separately from the
controller 40, for communications with the transmitter 50.
[0061] FIG. 2A shows an example system 200 of the type shown in
FIG. 1. The system 200 is shown as having been implanted in a
patient, and in addition, a programmer 270 is also shown that is
external to the patient. As shown, the system 200 is of a type that
is entirely implantable. The system 200 includes several seed
electrode assemblies 220, there being four such assemblies shown as
having been implanted within the heart 230 in FIG. 2A. The system
200 also includes an implantable combined pacing controller and
transmitter device 240 that has an antenna 260 for communicating,
for example, to the seeds 220. The controller/transmitter device
240 is shaped generally elongate and slightly curved so that it may
be anchored between two ribs of the patient, or possibly around two
or more ribs. In one example, the controller/transmitter device 240
is 2 to 20 cm long and 1 to 10 centimeters (cm) in diameter,
preferably 5 to 10 cm long and 3 to 6 cm in diameter. Such a shape
of the controller/transmitter device 240, which allows the device
240 to be anchored on the ribs, allows an enclosure that is larger
and heavier than conventional pacemakers, and allows a larger
battery having more stored energy. Other sizes and configurations
may also be employed as is practical.
[0062] The antenna 260 in the FIG. 2A example is a loop antenna
comprised of a long wire whose two ends 270 and 272 extend out of
the housing of the controller/transmitter device 240 at one end 280
of the controller/transmitter device 240. The opposite ends 270 and
272 of the loop antenna 260 are electrically connected across an
electronic circuit contained within the controller/transmitter
device 240, which circuit delivers pulses of RF current to the
antenna, generating a magnetic field in the space around the
antenna to charge the seeds, as well as RF control magnetic field
signals to command the seeds to discharge. The loop antenna 260 may
be made of a flexible conductive material so that it may be
manipulated by a physician during implantation into a configuration
that achieves improved inductive coupling between the antenna 260
and the coils within the implanted seeds 220. In one example, the
loop antenna 260 may be 2 to 22 cm long, and 1 to 11 cm wide,
preferably 5 to 11 cm long, and 3 to 7 cm wide. Placement of the
antenna over the ribs allows a relatively large antenna to be
constructed that has improved efficiency in coupling RF energy to
the pacing seeds.
[0063] In FIG. 2A, the loop antenna 260 has been configured to
extend generally around the periphery of the housing of the
controller/transmitter device 240. In particular, the loop antenna
260 extends from its first end 270 (located at the first end 280 of
the controller/transmitter device 240) outwardly and then generally
parallel to the elongately shaped controller/transmitter device 240
to the second end 282 of the controller/transmitter device 240.
From there, the loop antenna 260 extends outwardly and again
generally parallel to the controller/transmitter device 240, albeit
on an opposite side of the transmitter/controller device 240, and
back to the first end 280 of the controller/transmitter device 240.
As such, the loop antenna 260 may, like the controller/transmitter
device 240, be anchored to the ribs of the patient.
[0064] In this configuration, the distance between the center of
the loop antenna 260 and the seed electrode assemblies 220 will
typically be, on average, about three inches (3''). As will be
shown later, such a distance puts significant power demands on the
controller/transmitter device 240, and so an internal battery
included within the controller/transmitter device 240 may need to
be rechargeable. In some embodiments, however, the
controller/transmitter device 240 may be non-rechargeable. The loop
antenna 260 may have a shape that is more complex than that shown
in FIG. 2, with a larger antenna area, or multiple antenna lobes to
capture more tissue volume. The antenna may consist of two or more
wire loops, for example, one on the front of the patient's rib
cage, and a second on the back, to gain magnetic field access to a
larger tissue region.
[0065] Referring to FIG. 2B, there is shown an embodiment as shown
in FIG. 2A, but which also includes a conventional pacemaker, or
pulse generator, 290 and associated wired leads 295 which extend
from the pulse generator 290 and into chambers of the heart 600. As
such, the pulse generator 290 may be used to sense the internal ECQ
and may also communicate with the controller/transmitter 240 as
discussed previously.
[0066] Referring to FIG. 3, an embodiment of the
controller/transmitter 240 and associated loop antenna 260 is shown
in block diagram form. Included within the pacing controller 240
is: a battery 302, which may be recharged by receiving RF energy
from a source outside the body via antenna 260; ECG sensing
electrodes 304 and associated sensing circuitry 306; circuitry 308
for transmitting firing commands to the implanted seeds,
transmitting status information to the external programmer,
receiving control instructions from the external programmer and
receiving power to recharge the battery; and a controller or
computer 310 that is programmed to control the overall functioning
of the pacing control implant. In alternative embodiments, antenna
260 may receive signals from the individual seeds 220 containing
information regarding the local ECG at the site of each seed,
and/or antenna 260 may receive signals from a more conventional
implanted pacemaker regarding the ECG signal at the sites of one or
more conventional leads implanted on the right side of the
heart.
[0067] FIG. 4 is a schematic diagram of an exemplary wireless
electrode assembly, or seed, 420 that may serve as the seeds 20 or
220 as shown in either FIG. 1 or FIGS. 2A-B. The seed 420 includes,
firstly, a receiver coil 410 that is capable of being inductively
coupled to a magnetic field source generating a time-varying
magnetic field at the location of coil 410, such as would be
generated by the transmitter 50 and the antenna 60 shown in FIG. 1.
The RF current in the external antenna may be a pulsed alternating
current (AC) or a pulsed DC current, and thus the current induced
through the receiver coil 410 would likewise be an AC or pulsed DC
current. The current induced in coil 410 is proportional to the
time rate of change of the magnetic field generated at the site of
coil 410 by the external RF current source. A four-diode bridge
rectifier 415 is connected across the receiver coil 410 to rectify
the AC or pulsed DC current that is induced in the receiver coil
410. A three-position switch device 418 is connected so that when
the switch device 418 is in a first position, the rectifier 415
produces a rectified output that is imposed across a capacitor 405.
As such, when the switch device 418 is in the position 1 (as is the
case in FIG. 4), the capacitor 405 stores the induced electrical
energy.
[0068] The switch device 418, in this example, is a
voltage-controlled device and is connected to sense a voltage
across the capacitor 405 to determine when the capacitor 405 has
been sufficiently charged to a specified pacing threshold voltage
level. When the capacitor 405 is sensed to have reached the
specified pacing threshold level, the voltage-controlled switch
device 418 moves to a position 2, which disconnects the capacitor
405 from the coil 510. With the switch device 418 in the position
2, the capacitor 405 is electrically isolated and remains charged,
and thus is ready to be discharged. The voltage controlled switch
device 418 may consist of a solid state switch, such as a field
effect transistor, with its gate connected to the output of a
voltage comparator that compares the voltage on capacitor 405 to a
reference voltage. The reference voltage may be set at the factory,
or adjusted remotely after implant via signals sent from the
physician programmer unit, received by coil 410 and processed by
circuitry not shown in FIG. 4. Any electronic circuitry contained
within the seed, including the voltage controlled switch, is
constructed with components that consume very little power, for
example CMOS. Power for such circuitry is either taken from a
micro-battery contained within the seed, or supplied by draining a
small amount of charge from capacitor 405.
[0069] A narrow band pass filter device 425 is also connected
across the receiver coil 410, as well as being connected to the
three-position switch device 418. The band pass filter device 425
passes only a single frequency of communication signal that is
induced in the coil 410. The single frequency of the communication
signal that is passed by the filter device 425 is unique for the
particular seed 20 as compared to other implanted seeds. When the
receiver coil 410 receives a short magnetic field burst at this
particular frequency, the filter device 425 passes the voltage to
the switch device 418, which in turn moves to a position 3.
[0070] With the switch device in the position 3, the capacitor 405
is connected in series through two bipolar electrodes 430 and 435,
to the tissue to be stimulated. As such, at least some of the
charge that is stored on the capacitor 405 is discharged through
the tissue. When this happens, the tissue becomes electrically
depolarized. In one example embodiment that will be shown in more
detail later, the bipolar electrodes 430 and 435 across which
stimulation pulses are provided are physically located at opposite
ends of the seed 420. After a predetermined, or programmed, period
of time, the switch returns to position 1 so the capacitor 405 may
be charged back up to the selected threshold level.
[0071] It should be noted that, for sake of clarity, the schematic
diagram of FIG. 4 shows only the seed electrical components for
energy storage and switching. Not shown are electronics to
condition the pacing pulse delivered to the tissues, which
circuitry would be known to persons skilled in the art. Some
aspects of the pulse, for example pulse width and amplitude, may be
remotely programmable via encoded signals received through the
filter device 425 of the seed 420. In this regard, filter 425 may
be a simple band pass filter with a frequency unique to a
particular seed, and the incoming signal may be modulated with
programming information. Alternatively, filter 425 may consist of
any type of demodulator or decoder that receives analog or digital
information induced by the external source in coil 410. The
received information may contain a code unique to each seed to
command discharge of capacitor 405, along with more elaborate
instructions controlling discharge parameters such as threshold
voltage for firing, duration and shape of the discharge pulse,
etc.
[0072] Using seeds of the type shown in FIG. 4, all of the
implanted seeds may be charged simultaneously by a single burst of
an RF charging field from a transmitter antenna 60. Because back
reaction of the tiny seeds on the antenna 60 is small, transmitter
50 (FIG. 1) losses are primarily due to Ohmic heating of the
transmit antenna 60 during the transmit burst, Ohmic heating of the
receive coil 410, and Ohmic heating of conductive body tissues by
eddy currents induced in these tissues by the applied RF magnetic
field. By way of comparison, if eight seeds are implanted and each
is addressed independently for charging, the transmitter 50 would
be turned ON eight times as long, requiring almost eight times more
transmit energy, the additional energy being primarily lost in
heating of the transmit antenna 60 and conductive body tissues.
With the seed 420 of FIG. 4, however, all implanted seeds are
charged simultaneously with a burst of RF current in antenna 260,
and antenna and body tissue heating occurs only during the time
required for this single short burst. Each seed is addressed
independently through its filter device 425 to trigger pacing. The
transmitted trigger fields can be of much smaller amplitude, and
therefore lose much less energy to Ohmic heating, than the
transmitted charging pulse.
[0073] FIG. 5 is a flowchart of a pacing cycle that shows such a
mode of operation of charging all implanted seeds 20
simultaneously, and triggering the discharge of each seed 20
independently. The method starts at step 510 with the start of a
charging pulse that charges all of the seeds simultaneously. When a
pacing threshold voltage is attained or exceeded, at step 520, the
seeds switch to a standby mode (for example, switch 418 in seed 420
moves to position 2). Next, in step 530, at the appropriate time, a
controller/transmitter device such as device 240 shown in FIG. 2,
transmits a trigger pulse at a particular frequency (f1) that is
passed through a band pass filter (such as filter device 425) in
the seed to be fired (for example, seed 1). Then, at step 540, that
seed, namely seed 1, receives the trigger pulse through the band
pass filter, which in turn trips the switch to pace the tissue.
This process may be repeated for each of the N number of seeds that
have been implanted, as indicated at step 550, which returns to
step 530 where there are additional seeds that have been charged
and are to be fired. Next, at step 560 there is a delay until the
next diastole, after which time the process begins anew at step
510. The exact time of firing of the first seed may be programmed
by the physician in relation to the ECG signal features measured by
the sensing electrodes 304 in FIG. 3, or in relation to ECG
information transmitted to the controller 240 by the pacing seeds
themselves, or in relation to pacing information transmitted to the
controller 240 by a conventional implanted pacemaker, or in
relation to pacing information received from a conventional
implanted pacemaker through an implanted hard wire connection to
controller 240. Subsequent timing of the firing of each additional
seed may be programmed by the physician at the time of implant.
Note that seeds may be programmed not to discharge. For example, an
array of seeds may be implanted, but only a subset may be
programmed to receive firing commands from the controller 240.
[0074] In the case of FIG. 2A and other similar embodiments, it is
envisioned that the controller/transmitter device 240 and
associated antenna 260 would first be implanted subcutaneously in a
designed location (for example, between the ribs in the case of the
FIG. 2A embodiment). The physician then may program the
controller/transmitter 240 by delivering telemetric signals through
the skin using the programmer 270 in a conventional manner,
although this programming may also be done, at least in part,
before implantation. One of the adjustable parameters is the timing
of firing of each seed 220, determined by a time at which a short
burst of current at the frequency for the particular seed 220 is
delivered to the antenna 260. The controller/transmitter device 240
may have a pair of sensing electrodes on its surface to detect the
subcutaneous electrocardiogram (ECG), or it may contain multiple
electrodes to provide a more detailed map of electrical activity
from the heart. This local ECG signal sensed by the
controller/transmitter device 240 may be used to trigger the onset
of seed pacing when the patient has a functioning sinus node. In
any case, the signals sensed by the controller/transmitter device
240 are used to monitor ECG signals from the paced heart. In some
cases, these ECG signals, or other physiologic sensor input
signals, may be used to adjust or adapt the timing of firing of the
pacing seeds 220.
[0075] Alternatively, the controller 240 may receive local ECG or
pacing information through an RF link from a conventional pacemaker
290 implanted in the pectoral region of the patient, as shown in
FIG. 2B. This may be desirable in patients who already have a
conventional pacemaker, or when local ECG data from the
conventional atrial or right ventricular apex pacing sites are
desired to coordinate the timing of firing of the implanted seeds
220. Finally, the seeds 220 could themselves transmit information
to controller 240 concerning the local bi-polar ECG measured at
their sites. Alternatively, the seeds 220 could sense the local ECG
and discharge based upon this local data, with no firing
instructions from the controller 240 required, or the seeds 220
could transmit information from seed 220 to seed concerning local
ECG and onset of their discharge. All of the above embodiments, a
combination, or a subset, may be implemented in this invention.
[0076] In an example embodiment, the seeds 220 would be delivered
to their respective sites in the cardiac veins, within the heart
wall, or on the epicardial surface of the heart via a catheter, as
will be described in more detail later. A distal portion, or tip of
the catheter, may contain a single electrode or a pair of
electrodes, each being connected to a signal recorder via leads
extending to a proximal end of the catheter. As such, it is
possible to obtain a uni-polar or bipolar ECG at the catheter
distal tip. The physician would select the implantation site based
upon features of the ECG signal sensed using the catheter. The seed
then may be injected through a needle extended from the catheter
tip, or it may be pushed into the tissue and then released from the
catheter. Many mechanisms may be used for seed release, including
the release or addition of fluid pressure to the catheter tip.
[0077] Once implanted, the seed 220 may be charged and then fired
to observe the altered electrogram proximate the seed at the
location of the catheter tip. The physician can adjust the timing
of seed firing by programming the controller/transmitter device
240. When satisfied with the local and controller/transmitter
device 240 electrograms, the catheter (or a seed delivery mechanism
residing within the catheter) may be removed, and a new delivery
mechanism containing the next pacing seed may be inserted and
navigated to the next pacing site. Because seeds can be fired in
any order, or not fired at all, a physician may deliver the seeds
in any order. When the heart is deemed to be beating in synchrony,
no further seeds need be implanted. Alternatively, if it has been
determined that the seeds are small enough that they do not
substantially impair local tissue function, then an array of seeds
may be delivered to the veins and/or heart wall, and the physician
can program a subset of seeds to fire in a sequence that optimizes
the pumping efficiency of the heart. Ejection fraction and cardiac
output may be measured to determine pumping efficiency. On any
given heartbeat, some or all of the seeds would fire. The
controller 240 may be programmed to sequentially fire seeds, or
some seeds may fire simultaneously.
[0078] FIGS. 6-10 show an example of a mechanical design for a seed
electrode assembly and an example seed delivery device and method.
Referring first to FIG. 6, a system of the type shown in FIG. 2 is
shown where three seed electrode assemblies 220 have been implanted
within tissue of the heart 600, and in particular, within a
myocardial wall 605 of the heart 600. In addition, the
controller/transmitter device 240 is shown implanted beneath the
skin 610 of the patient. The antenna 260 extends from within the
controller/transmitter device 240 at one end of the device 240, and
then extends around the periphery of the device 240, as described
previously. The external programming device 270 is also shown,
which is used to communicate with the implanted
controller/transmitter 240.
[0079] Distal portions of two seed delivery catheters 615 are shown
in FIG. 6, each extending within a chamber of the heart 600 and to
a site near where one of the seeds 220 is located. Generally, the
delivery catheter 615 enables placement of a seed 220 and the
ability to sense the electrical activity at the distal tip of
delivery catheter 615 through catheter tip electrode 625, so that a
physician can determine if the location is a good candidate
location for implantation of seed 220. If the location is a good
candidate, the seed 220 may be partially inserted into the tissue
as shown in FIG. 9. With the seed 220 still tethered to a pull wire
735A, the seed 220 may be charged and then discharged into the
tissue, while the physician observes electrograms, including the
local electrogram arising from electrode 625, and perhaps an
electrogram from the distal seed electrode taken through the pull
wire 735A. Upon firing the seed, if the physician determines it is
not in the proper location to optimize cardiac output, then the
seed 220 may be removed from that site and positioned elsewhere. If
it is an appropriate location, then the seed 220 has an anchoring
mechanism that can be activated to implant the seed 220 permanently
within the tissue so that it retains its location.
[0080] Each of the catheters 615 is shown in FIG. 6 extending into
the heart 600 through a heart entry vessel 620 such as the inferior
vena cava (for right chamber entry) or aortic valve (for left
chamber entry). A distal portion 625 of the delivery catheter 615
includes a sensing electrode for sensing the electrical activity at
a tissue site where the seed 220 may be implanted.
[0081] FIG. 7 shows one of many possible embodiments of a wireless
electrode assembly, or seed, 220. The seed 220 is shown in FIG. 7
within a distal portion of the seed delivery catheter 615. The seed
220 has a main body 702 that, in this example, is bullet shaped and
has two bipolar electrodes 705 and 710. One of the electrodes,
namely electrode 705, is located at a distal tip of the
bullet-shaped seed body 702, and the other electrode 710 is located
at a proximal end of the seed body 702. The bullet shape of the
seed body 702 enables it to be extended into tissue such as the
myocardial wall 605, as will be illustrated in later figures. In
other embodiments, the "nose," or distal tip, of the seed body 702
may be more cone-shaped than the embodiment shown in FIG. 7. While
the distal and proximal electrodes 705 and 710 are shown on the
seed itself, other locations are possible, including placing the
distal and proximal electrodes 705 and 710 at the ends of the
attachment tines to achieve the maximum separation between
electrodes.
[0082] The seed delivery catheter 615 consists of an elongate tube
with a main lumen 712 extending though its entire length. The
catheter 615 has an opening 713 at its distal end so that the seed
220 may be released from the delivery catheter 615. The catheter
615 also has the previously discussed electrode 625, which as shown
extends around the periphery of the distal opening 713. An
electrically conductive lead 716 is attached to the electrode 625
and extends proximally through the entire length of catheter lumen
712, or through the wall of the catheter, and outside the body (not
shown in FIG. 7). The lead 716 is made of an electrically
conductive material, and thus provides the local electrocardiogram
(ECG) appearing at the distal electrode 625. As such, the
electrical activity appearing at the location of the distal seed
electrode 705 may be viewed external of the patient to determine if
that is an appropriate location to implant the seed 220.
[0083] By way of example, the main lumen 712 of the seed delivery
catheter 615 may have an internal diameter of about two-and-a-half
millimeters, and the seed delivery catheter 615 may have an outside
diameter that is slightly larger than that. In this case, the seed
body 702 may have a width of about two millimeters, and the length
of the seed body 702 may be about five to ten millimeters, for
example. This enables the seed 220 to be implanted entirely within
a myocardial wall 605, which may, for example, be about 20
millimeters thick in the left ventricle.
[0084] The seed 220 has a pair of forward-end tines 715 and 720
that each extend from a common junction point 725. Each of the
tines 715 and 720 may be about three to eight millimeters in
length, for example. The seed body 702 also has a central bore 730
extending longitudinally through a center of the seed body 702. In
FIG. 7, which shows the seed 220 not yet implanted, one of the
forward-end tines, namely tine 720, extends proximally into the
bore 730, while the other forward-end tine 715 extends distally to
enable it to pierce through tissue. As will be described in more
detail later, the junction point 725 for the tines 715 and 720 may
be pushed forward of the seed 220 body, and when the constrained
tine 720 clears the central bore 730, the tines 720 and 715 are
biased to snap into a lateral configuration that will be shown in a
later figure. The junction point 725 is physically larger than the
diameter of the central bore 730, and thus enables the seed 220 to
be pulled in a proximal direction by pulling on extraction wire
735.
[0085] The seed extraction wire 735 is attached to the junction
point 725, and extends proximally through the entire length of the
seed central bore 730, and from there continues proximally through
the delivery catheter 615 and outside the body (not shown in FIG.
7). The wire 735 may be made of an electrically conductive material
so as to sense an electrical signal appearing at a distal end of
the wire 735, thus serving as an extraction pull wire and as a
temporary ECG lead for distal electrode 705. This is a means of
sensing a bipolar electrocardiogram at a proposed implantation site
before permanently implanting the seed 220, using electrode 705
(with wire lead 735) as a first electrode, and using the catheter
electrode 625 and lead 716 as a second electrode.
[0086] In that the extraction wire 735 extends outside the
patient's body, a physician may pull the wire 735, and given that
the junction point 725 is too large to be pulled into the seed body
central bore 730, pulling the wire 735 pulls the seed 220
proximally within the delivery catheter 615. The extraction wire
735 is also constructed of a material and of a diameter such that
the wire 735 is rigid enough to be pushed forward to extend the
junction point 725 forward of the seed 220 body and hence free the
forward-end tine 720 from the constraining central bore 730. The
wire 735 has stopper device 740 that is attached to the wire 735 at
a point that is proximal of the seed 220 body. The stopper device
740, like the junction point 725, is larger than the seed body
central bore 730, and thus constrains how far the lead junction
point 725 can be extended forward of the seed body 702. The stopper
device 740 is positioned on the wire 735 at a location that is far
enough away from the rear-end of the seed body 702 such that wire
735 may be pushed distally far enough to free the constrained tine
720 from the seed body central bore 730.
[0087] The extraction wire 735 has a detachment mechanism 745
located on the wire 735 at a point that is immediately distal of
the stopper device 740. The detachment mechanism 745 may be
activated by a physician to detach the portion of wire 735 that is
proximal of the detachment mechanism 745. Various detachment
mechanisms may be used for the detachment mechanism 745. For
example, the detachment mechanism 745 may be a high-resistance
portion of a conductive line that extends proximally to a point
external of the patient, and that can be heated and detached by
injecting current of a specified amount into the conductive line.
In this case the wire 735 may serve three purposes: extraction of a
seed 220 from a location that does not provide optimal cardiac
resynchronization; conduction of the tip electrode 705 ECG signal
to a recorder outside the body; conduction of a burst of current to
detach itself at a point 745 of relatively high electrical
resistance. Another example for the detachment mechanism 745 is a
mechanical configuration where the proximal detachable portion of
the lead 735 may be unscrewed from the remainder of the lead 735,
or where the lead 735 is pushed and turned in a certain way to
effect detachment of the proximal portion from the remainder of the
lead 735. A mechanical skiving or shearing means (not shown) may
alternatively be applied at point 745.
[0088] The seed 220 also has a pair of tines 750 and 755 that
extend from the rear end of the seed body 702. In the shown
example, there are two such tines 750 and 755, though it will be
understood that there may be more than two tines, or a single tine.
The tines 750 and 755 assist in securing the seed 220 at a desired
location within the tissue, such as within a desired location of
the myocardial wall 605, to prevent the seed from migrating under
the repeated stress of heart muscle contraction. The tines 750 and
755, in this example, are attached to the rear-end electrode 710
near a periphery of the electrode 710, and extend from their
attachment points in a direction that is about 45 degrees from a
longitudinal axis of the seed body 702. As shown in FIG. 7,
however, far ends of the tines 750 and 755 are constrained by an
outer wall of the catheter lumen 712, and become bent toward the
longitudinal axis of the catheter 615. When the seed 220 is pushed
out of the distal end of catheter 615, the tines 750 and 755 spring
outwardly into their normal position (not shown in FIG. 7).
[0089] A tube 760 that is movable longitudinally within the
catheter 615 is used to push the seed 220 distally within the
catheter 615 and out of the catheter distal opening 713. The tube
has a lumen 765 extending longitudinally through its entire length
so that the wire 735 extends through the tube lumen 765. The
cross-sectional diameter of the pusher tube 760 may be, for
example, about half that of the catheter lumen 712. As such, where
the catheter lumen 712 diameter is about 2.5 mm, the tube
cross-sectional diameter may be about 1.25 mm.
[0090] In FIG. 8, the seed delivery catheter 615, with a seed 220
contained within, is shown with its circular distal electrode 625
pressed against the myocardial wall 605. In the configuration
shown, it is possible for the electrical activity occurring at that
site of the myocardial wall 605 to be monitored at a proximal end
of the lead 716 to determine if the site is an appropriate
candidate site in which to implant the seed 220.
[0091] Turning now to FIG. 9, two seeds 220A and 220B are shown.
The first seed 220A is shown during the process of implanting the
seed 220A within the myocardial wall 605, with the assistance of
the seed delivery catheter 615. The second seed 220B is shown as
having already been permanently implanted within the myocardial
wall 605.
[0092] The first seed 220A is shown as having been pushed nearly
entirely within the myocardial wall 605. This was accomplished by
the physician pushing the push tube 760 within the seed delivery
catheter 615 so as to push the seed 220A out of the catheter's
distal opening 713. The forwardly extending distal tine 715 served
to pierce the myocardial wall 615 and permit implantation within
the wall 615.
[0093] In the position shown in FIG. 9, the seed's rear-end tines
750A and 755A are still partially within the seed delivery catheter
615 and thus are still being constrained from extending outwardly
from the seed body's longitudinal axis. As such, it is still
possible for the physician to pull back the seed 220A from this
position by pulling on the seed extraction wire 735A. If the seed
220A were to have been pushed a little further so that the proximal
tines 750A and 755A become extended, then it may not be possible to
pull back the seed 220A. As discussed previously, seed 220A may be
charged and commanded to discharge while wire 735 serves as a lead
to monitor the electrical activity at the forward end of the seed
220A. The physician may determine that the present positioning is
not appropriate, and wire 735 may then be pulled to extract the
seed, which may then be moved to an alternate location.
[0094] Also in the position shown in FIG. 9, the wire 735 has not
yet been pushed forward to deploy the distal tines 715A and 720A
(750A not shown in FIG. 9). Deploying the distal tines 715A and
720A is done as follows. First, the pushing tube 760 is used to
push the seed 220A so that, firstly, the proximal tines 750A and
755A are freed from the delivery catheter 615 and thus extend
outwardly, and secondly, the seed's distal tine junction point 725A
extends distally of the seed, and preferably entirely through the
myocardial wall 605. In particular, the junction point 725A and one
of the forward-end tines 715 are both positioned outside the
myocardial wall 605 in FIG. 9. Next, the wire 735A is pushed
distally until the lead stopper device 740 becomes flush with the
proximal seed electrode 710A. When this occurs, the constrained
tine 720A becomes removed from the seed body central bore, thus
allowing the two distal tines 715A and 720A to pop into the lateral
position. Seed 220B is shown in the deployed position, the proximal
tines 750B and 755B are shown extended, and the two distal tines
715B and 720B are outside the myocardial wall 605 and extend
laterally from the junction point 725B.
[0095] Referring now to FIG. 10, a flowchart is shown that
describes a method of delivering a seed 220 using the catheter 615
or another similar delivery device. The method begins at step 1010
with the percutaneous transluminal delivery of the catheter 615 to
the heart chamber. This may be accomplished in the following
manner. First, an introducer is used to provide entry into, for
example, the femoral vein or artery (depending on where the seed
220 is to be delivered). The catheter 615 is then inserted so that
its distal end is snaked through the inferior vena cava and into
the right atrium, for example. Thus, a seed 220 may be delivered in
the right atrium. The distal end of the catheter 615 may also be
moved from the right atrium, through the tricuspid valve, and into
the right ventricle, for delivery of a seed 220 there. The distal
end of the catheter may also be pushed through the fossa ovalis,
accessed on the right atrial septum, for placement of seeds 220 in
the left heart chambers. Alternatively, the distal end of the
catheter 615 may be snaked through the femoral artery and
descending aorta, through the aortic valve and into the left
ventricle, and from the left ventricle may be moved through the
mitral valve into the left atrium. Navigating the catheter 615 may
require that the catheter 615 have some type of navigational
capability such as push and pull wires commonly used with
electrophysiology catheters.
[0096] Next, at step 1020, a sample ECG signal may be taken at
sites on the heart inner wall. This may be done with the catheter
615 positioned as shown in FIG. 8, for example. At step 1030, the
physician selects a site at which to deliver the seed 220. Then, at
step 1040, the physician delivers the seed 220 into the myocardial
wall tissue, such as shown with seed 220A in FIG. 9. At this point,
the seed 220 is still tethered by the lead 735A so that the seed
may be pulled back into the delivery catheter 615 if necessary.
Further at step 1040 a test pace is performed to test the response
at this site. This may be done using the programmer 270 shown in
FIG. 6 to instruct the controller/transmitter device 240 to send a
charging signal and then a trigger signal to the particular seed
220.
[0097] If the pacing response is found, at step 1050, to be
unacceptable, then the seed 220 may be removed and the process may
be performed again starting at step 1020. If, on the other hand,
the pacing response is found to be acceptable, then, at step 1060,
the anchoring means for the seed 220 may be activated, for example,
by moving the seed 220 entirely out of the catheter 615 and freeing
the proximal tines 750 and 755 from the constraints of the catheter
615 and pushing the lead 735 to release the distal tines 715 and
720. Also at step 1060, the tether to the seed 220 may be released,
for example, using the detachment mechanism 745. Having completed
the implantation of the seed, it is now possible at step 1070 to
begin placement of the next seed 220.
[0098] As discussed previously, each of the seeds 220 may have a
filter 425 (see FIG. 4) that allows passage of a signal of a
particular frequency. Thus, for example, where eight seeds 220 are
implanted, each of the seeds 220 may have a band pass filter 425 of
a different center frequency. To make this possible, seeds 220 may
be manufactured as having one of sixteen different band pass
frequencies. Thus, up to sixteen seeds 220 may be implanted so that
each seed is separately controllable. A code for the particular
pass frequency may be labeled directly on the seed 220 itself, or
alternatively, may be labeled on the packaging for the seed 220. As
such, when programming the system 200 using the programmer 270, the
particular band pass frequency for each seed 220 is communicated to
the pacing controller 240.
[0099] A variety of alternative embodiments are envisioned for seed
delivery and detachment. For example, FIG. 11A shows a seed 1120A
that is secured into the myocardium 605 with a distal spring 1105A,
or "cork screw." A delivery rod 1110 provided by a delivery
catheter 1112 is detached from the seed 1120A by turning the rod
1110 to engage the spring into tissue and also unscrew the threaded
distal rod section 1115 from the seed 1120A. In FIG. 11B, a distal
spring 1105B is screwed into the myocardium 605 using a clockwise
rotation of the seed 1120B, which also unscrews the delivery rod
from the seed. Upon removal of the delivery rod, proximal spring
1125 is exposed to the myocardium 605. Clockwise spring 1105B and
counter-clockwise spring 1125 together prevent rotation and
translation of the seed through the myocardium. A mechanism for
release of the springs is not shown in the figure. A small push rod
passing through the delivery rod and seed could be used to push the
distal spring from the seed and into a locked position. A thin
sheath could cover proximal spring 1125. The thin sheath would be
retracted along with the delivery rod. Alternate means for
detachment of the delivery rod include Ohmic heating of a high
resistance portion of the rod, and mechanical shearing. In FIGS.
11C-D, tines 1130 are pushed, using a push rod 1135 provided
through the main lumen of the delivery catheter 1112, from the
central portion of the seed 1120C, out through channels 1140 and
into the myocardium 605, so that the tines 1130 extend laterally
from the seed 1120C body (as shown in FIG. 11D), and so that the
seed 1120C becomes secured within the tissue. The push rod 1135 is
removable, at an attachment point, from a proximal end junction
point 1145 of the tines 1130. Various mechanisms for removing, or
detaching the push rod 1135 from the tine proximal end junction
point 1145 may be employed, as discussed previously in connection
with the FIG. 7 embodiment.
[0100] Referring now to FIGS. 11E-K, some embodiments that are
envisioned for seed delivery and detachment include a seed 1120E
having a helical tine 1105E and one or more adjustable tines 1110E
that secure the seed 1120E to the myocardium 605. In such
embodiments, detachment mechanisms 1145E and 1165E may be used to
release the seed 1120E from an elongate shaft 1160E after the seed
1120E is secured to the myocardium 605.
[0101] Referring to FIG. 11E, the seed 1120E is shown within a
distal portion of the seed delivery catheter 615. The seed 1120E
has a main body 1122E that, in this example, is cylindrically
shaped with a tip portion 1123E at a distal end. The seed 1120E may
include two bipolar electrodes 1135E and 1136E that are capable of
discharging an electrical pulse. Electrode 1135E is located at the
distal end of seed body 1122E, and the other electrode 1136E is
located at a proximal end of the seed body 1122E. In this
embodiment, the tip portion 1123E of the seed body 1122E has a
modified cone shape that facilitates delivery of the distal end of
the seed 1120E into tissue such as the myocardial wall 605, as will
be illustrated in later figures. The tip portion 1123E may serve as
a strain relief mechanism for the adjustable tines 1110E that
extend from the tip portion 1123E. Furthermore, the tip portion
1123E may also deliver a steroid elution to minimize the formation
of fibrous tissue at the seed/myocardium interface. While the
distal and proximal electrodes 1135E and 1136E are shown on the
seed body itself, other locations are possible. For example, the
distal electrode 1135E may be disposed at the end of the helical
tine 1105E to achieve the maximum separation between electrodes, or
may be an entire tine. In another example, the surface of tip
portion 1123E on the seed body 1122E may function as the distal
electrode 1135E, which may provide a more efficient use of space
when the seed body 1122E is substantially smaller in size.
Furthermore, using the surface of tip portion 1123E to function as
the distal electrode 1135E may be desirable in circumstances where
only the tip portion 1123E contacts the endocardium or myocardium
tissue (described in more detail below).
[0102] As previously described, the seed delivery catheter 615
includes an elongate tube with a main lumen 712 extending though
its entire length. The catheter 615 has an opening 713 at its
distal end so that the seed 1120E may be released from the distal
end of the delivery catheter 615. In some circumstances, all or a
portion of the seed 1120E may extend from the delivery catheter 615
before the seed 1120E is secured to the heart tissue. In those
cases, the main lumen 712 may still be sized to slidably engage the
elongate shaft. The catheter 615 may also have an electrically
conductive lead 716 and an electrode 625 that extends around the
periphery of the distal opening 713 and is capable of providing
local ECG information as previously described. In some embodiments,
it may be necessary to secure the tip of the catheter 615 to the
heart tissue during seed placement. For example, the distal end of
the catheter 615 may include a screw mechanism to temporarily
secure the catheter 615 to the heart tissue (described in more
detain below in connection with FIG. 13).
[0103] In this embodiment, the seed 1120E has a plurality of
adjustable tines 1110E that each extend from a common junction
member 1112E. As shown in FIG. 11E, each of the adjustable tines
1110E generally extend from the junction member 1112E through a
central bore 1130E of the seed body 1122E. FIG. 11E shows the seed
1120E not yet implanted, and only the helical tine 1105E extends
from the seed body 1122E while the adjustable tines 1110E are
disposed in the central bore 1130E. As will be described in more
detail later, the junction member 1112E may be pushed in a distal
direction by an actuation rod 1170E, thereby forcing the adjustable
tines 1110E from the distal end of the central bore 1130E. When the
constrained tines 1110E extend from the central bore 1130E, the
tines 1110E are biased to extend in a curled or hook configuration.
The junction member 1112E may be physically larger than the
diameter of the central bore 1130E, providing a stopping point for
actuation of the adjustable tines 1110E.
[0104] Still referring to FIG. 11E, the elongate shaft 1160E
includes a detachment mechanism 1165E at a distal end that is
capable of engaging/disengaging the detachment mechanism 1145E of
the seed 1120E. In this embodiment, the detachment mechanism 1165E
includes a threaded member that engages a complementary threaded
member on the seed's detachment mechanism 1145E. The threaded
engagement between the detachment mechanisms 1165E and 1145E may be
arranged so that the threads would not release when the seed 1120E
is being advanced into the tissue with the rotation of the helical
tine 1105E.
[0105] From the detachment mechanism 1165E, the elongate shaft
1160E continues proximally through the delivery catheter 615 and
outside the patient's body (not shown in FIG. 11E). In that the
elongate shaft 1160E extends outside the patient's body, a
physician may direct the seed body 1122E (via the elongate shaft
1160E coupled thereto) through the lumen 712 of the delivery
catheter 615. (As described in more detail below in connection with
FIG. 11I, the delivery catheter 615 may be navigated through an
access catheter or other steerable sheath to the implantation site.
The access catheter is capable of maintaining a stable valve
crossing, which can reduce trauma to the valve and facilitate the
implantation of multiple seeds into the wall of the heart chamber.)
The elongate shaft 1160E may be constructed of a material and of a
size and design such that the elongate shaft 1160E is sufficiently
rigid to be rotated within the main lumen for purposes of engaging
the helical tine 1105E with the myocardium tissue. Also, the
elongate shaft 1160E may be sufficiently flexible so as to not
impede navigation of the elongate shaft 1160E and the catheter 615
to the implantation site.
[0106] The actuation rod 1170E may be disposed in a lumen 1162E of
the elongate shaft 1160E. The actuation rod 1170E includes an
engagement surface 1172E that is adapted to contact the junction
member 1112E. From the engagement surface 1172E, the actuation rod
1170E may continue proximally through the elongate shaft 1160E and
outside the patient's body. In such embodiments, a physician may
apply a force at the proximal end of the actuation rod 1170E so as
to slide the rod 1170E within the elongate shaft 1160E. Such motion
of the elongate rod 1170E may apply a distal force upon the
junction member 1112E. The actuation rod 1170E may be constructed
of a material and be of a size such that the actuation rod is
sufficiently rigid to push against the junction member 1112E and
force adjustable tines 1110E to extend from the distal end of the
central bore 1130E. Also, the elongate rod 1170E may be
sufficiently flexible so as to be guided through the lumen 1162E of
the elongate shaft 1160E.
[0107] Referring now to FIGS. 11F-11H, at least a portion of the
seed 1120E shown in FIG. 11E may be implanted into myocardium 605.
As previously described in connection with FIG. 6, the delivery
catheter 615 may be guided into a heart chamber (e.g., left atrium
32, left ventricle 34, right atrium 36, or right ventricle 38) to
enable placement of at least a portion of the seed 1120E from the
heart chamber into the myocardium 605. In such circumstances, the
seed may pass necessarily from the distal opening 713 of the
catheter 615, through an inner lining of the heart wall (e.g., the
endocardium 606), and into the myocardium 605. FIGS. 11F-11H show a
seed 1120E that is being implanted into the myocardium 605 and also
show a neighboring seed 1120E (below the first seed 1120E) that was
previously secured to the myocardium 605.
[0108] Referring to FIG. 11F, the seed 1120E in the lumen 712 of
the delivery catheter 615 may be directed toward the distal end by
a force 1167E from the elongate shaft 1160. The distal end of the
delivery catheter 615 may abut (or be positioned proximate to) the
inner surface of the heart chamber so that the seed 1120E is guided
to a selected site of the heart wall. As shown in FIG. 11E,
adjustable tines 1110E of the seed 1120E in the delivery catheter
615 are not in an actuated position where they extend from the
distal end of the central bore 1130E (the adjustable tines 1110E of
the neighboring seed 1120E that was previously implanted are shown
in an actuated position). The helical tine 1105E is configured to
penetrate through the endocardium 606 and into the myocardium 605,
as described in more detail below.
[0109] Referring to FIG. 11G, the seed 1120E in the lumen 712 of
the delivery catheter 615 may be rotated by a torsional force 1168E
from the elongate shaft 1160. By rotating the seed body 1122E along
its longitudinal axis, the helical tine 1105E may be "screwe" into
the heart wall. In such circumstances, the helical tine 1105E
penetrates through the endocardium 606 and into the myocardium 605.
In some embodiments where the detachment mechanism 1145E includes a
threaded member, the torsion force 1168E from the elongate shaft
1160E may serve to maintain or tighten the threaded engagement.
[0110] In the position shown in FIG. 11G, the seed's adjustable
tines 1110E are not extended from the central bore 1130E (as shown
by the neighboring seed). As such, it is still possible for the
physician to pull back the seed 1110E from this position by
rotating the elongate shaft 1160E in a direction opposite of force
1168E, which would cause the helical tine 1105E to "unscrew" from
the myocardium tissue. The seed's distal electrode 1135E is in
contact with the myocardium 605. As discussed previously, seed
1120E may be commanded to discharge a pacing electrical pulse while
electrode 625 on the delivery catheter 615 monitors the electrical
activity at the selected site. If the physician determines that the
present positioning of the seed 1120E is not satisfactory, the seed
1120E may be retracted into the delivery catheter lumen 712, which
may then be moved to an alternate location. At the alternate
location, the helical tine 1105E would again penetrate through the
endocardium and into the myocardium 605, in which case further
monitoring of electrical activity may occur.
[0111] Referring to FIG. 11H, after the seed 1120E is secured to
the heart wall (e.g., at least a portion of the helical tine 1105E
and perhaps a portion of the seed body 1122E is penetrated into the
endocardium) and after the physician determines that the
positioning of the seed 1120E is proper, the adjustable tines 1110E
may be forced to an actuated position. In this embodiment, the
actuation rod 1170E disposed in the elongated shaft 1160E is
capable of applying a force on the junction member 1112E. When the
junction member 1112E is forced toward the seed body 1122E, the
adjustable tines 1110E extend from the distal end of the central
bore 1130E. In this embodiment, the adjustable tines 1110E are
biased to have a curled or hook shape when unconstrained by the
central bore 1130E. For example, the adjustable tines 1110E may
comprise a shape memory alloy material, such as nitinol or the
like, that is capable of returning to its biased shape after being
elastically deformed within the central bore 1130E. The adjustable
tines 1110E embed in the myocardium 605 to provide supplemental
anchoring support and to substantially hinder additional rotation
of the seed body 1122E. As such, the elongate shaft 1160E may be
rotated backward relative to the seed body 1122E, which causes the
threaded members of detachment mechanisms 1165E and 1145E to
disengage one another. In this embodiment, the elongate shaft 1160E
may be rotated relative to the seed body 1122E without extracting
the seed 1120E from the myocardium 605 because the adjustable tines
1110E prevent the helical tine 1105E from being "unscrewed." After
the seed 1120E is detached from the elongate shaft 1160E, the
delivery catheter 615 and the elongate shaft 1160E may be withdrawn
from the implantation site.
[0112] In addition to preventing the seed body 1122E from
substantially rotating within the myocardium 605, the adjustable
tines also reduce the likelihood of the seed body 1122E being
pulled or torn from the heart wall. The seed 1120E may be exposed
to various forces from the beating heart and the turbulence of the
blood in the heart chambers. In some embodiments, the seed 1120E
may be attached to the heart wall so that a threshold amount of
pull force is required to remove the seed 1120E from the heart
wall. Certain embodiments of seed 1120E may be secured to the heart
wall such that a pull force of greater than 0.3 lbs. is required to
remove the seed body 1122E from the heart wall. In some
embodiments, the a seed 1120E may be secured to the heart wall such
that a pull force of greater than 0.5 lbs., and preferably greater
than 1.0 lbs., is required to remove the seed body 1122E from the
heart wall.
[0113] In one example, several seeds 1120E were secured to the
myocardium of a porcine (pig) heart using the helical tine 1105E
and three adjustable tines 1110E. The porcine heart was delivered
to a lab where a portion of it was removed by scalpel to reveal an
internal heart chamber. Several seeds 1120E were secured to the
porcine heart wall from the internal heart chamber--first by
rotating the helical tine 1105E into the myocardium and then by
actuating the adjustable tines 1110E to a curled shape
substantially within the myocardium tissue. Each of the seeds 1120E
was secured to the heart wall such that a pull force of greater
than 0.3 lbs. was required to remove the seed body 1122E from the
heart wall, and in some instances, a pull force of greater than 1.0
lbs. was required.
[0114] Referring now to FIG. 11I, helical tine 1105E and the
adjustable tines 1110E may secure the seed 1120E to the myocardium
605 such that at least a portion of the seed body 1122E (e.g., the
tip portion 1123E) penetrates into the myocardium 605. In some
embodiments where the seed 1120E is substantially smaller than the
myocardium wall thickness, the seed body 1122E may be fully
inserted into the myocardium tissue. In the embodiments described
in connection with FIGS. 11F-11H, a distal portion of the seed body
1122E extends into the myocardium 605 while a proximal portion of
the seed body 1122E is exposed to the heart chamber (e.g., left
atrium 32, left ventricle 34, right atrium 36, or right ventricle
38). As shown in those figures and in FIG. 11I, the seed body 1122E
may be secured to the myocardium 605 so that the distal electrode
1135E is in contact with the myocardium while the proximal
electrode 1136E is exposed to the heart chamber (and the blood
therein). In certain cases, such positioning of the seed body 1122E
may be dictated by a limited thickness in the myocardium wall.
[0115] Still referring to FIG. 11I, in some cases the seed body
1122E may not fully penetrate into the myocardium 605. For example,
as shown by the lower seed 1120E secured in the left ventricle 34
shown in FIG. 11, a portion of the seed 1120E (e.g., the helical
tine 1105E and the adjustable tines 1110E) may penetrate through
the endocardium while the a substantial portion of the seed body
1122E does not fully penetrate into the myocardium tissue. In such
circumstances, the tip portion 1123E may contact or penetrate into
the endocardium (and perhaps partially into the myocardium), but
the other portions of the seed body 1122E may not penetrate into
the heart wall. Yet in this position, the seed 1120E may be capable
of providing a pacing electrical pulse to the proximal heart
tissue. The delivery of the pacing electrical pulse may be
facilitated by using a surface of tip portion 1123E to function as
the distal electrode 1135E.
[0116] In some cases, such positioning of the seed body 1122E may
provide operational advantages. For example, if the distal
electrode 1135E is a cathode that generally depolarizes nearby
tissue cells, and if the proximal electrode 1136E is an anode that
may hyper-polarize nearby tissue cells, the position of the seed
body 1122E shown in FIGS. 11F-11I may reduce the effects of
hyper-polarization. Because, in this example, the anode is
generally exposed to blood in the heart chamber, the tissue cells
in the myocardium are not necessarily hyper-polarized by the anode.
In such circumstances, the pacing electrical charge between the
cathode, the nearby myocardium, the nearby blood in the heart
chamber, and the anode may reduce the hyper-polarization of local
areas in the myocardium tissue--a factor that may limit pacing
effectiveness.
[0117] Still referring to FIG. 11I, a distal end 676 of an access
catheter 675 may be guided to a heart chamber where the seed 1120E
is to be delivered. The access catheter 675 includes a lumen that
extends from a proximal end to the distal end 676. The access
catheter also includes a distal opening through which the delivery
catheter 615 slidably passes as it is directed to the selected site
proximal to the heart wall. In some embodiments, the access
catheter 675 may be used to establish and maintain a valve
crossing. In such circumstances, the delivery catheter 615 may be
fully withdrawn from the patient's body after a first seed 1120E
has been successfully implanted, yet the access catheter 675 can
maintain its position in the heart chamber. Then, a new delivery
catheter 615 and elongated shaft 1160E (with a second seed 1120E
attached thereto) may be guided through the access catheter 675 are
into the heart chamber. As shown in FIG. 11I, the access catheter
675 may approach the left ventricle 34 through the aorta (e.g.,
across the aortic valve and into the left ventricle 34). Other
approaches are contemplated, depending on the targeted heart
chamber, the conditions in the patient's heart vessels, the entry
point into the patient's body, and other factors. For example, the
access catheter 675 may approach the left ventricle 34 through the
inferior vena cava, through a puncture in the atrial septum, and
down through the mitral valve into the left ventricle 34.
[0118] As previously described, the delivery catheter 615 may
include a steering mechanism, such as push or pull wires, to aid in
placement of the distal end of the catheter 615 against a selected
site on the wall of the heart. Similarly, the access catheter 675
may include a steering mechanism, such as push or pull wires, to
aid in placement of the distal end 676 in the selected heart
chamber. In this embodiment, the access catheter 675 includes an
image device 685, such as an ultrasound probe or the like, proximal
to the distal end 676 of the access catheter 675. The image device
685 is capable of providing the physician with visualization of the
implantation site in the heart chamber. Because the inner surface
of the heart chambers may be substantially irregular in surface
topology as well as thickness, the image device 685 can be used by
a physician to visualize the implantation site and possibly measure
the myocardium wall thickness at that site. Such a feature may be
particularly advantageous where the procedure is to be conducted on
an active, beating heart.
[0119] Referring now to FIGS. 11J-11K, the adjustable tines 1110E
of the seed 1120E may be forced from a non-actuated position (e.g.,
FIG. 11J) to an actuated position (e.g., FIG. 11K). As previously
described, the seed 1120E may include a plurality of adjustable
tines 1110E. In this embodiment, the seed 1120E includes three
adjustable tines 1110E that each extend from the common junction
member 1112E. As shown in FIG. 11J, when the adjustable tines 1110E
are in a non-actuated position, the junction member 1112E is offset
from the seed body 1122E, and at least a portion of the adjustable
tines 1110E are constrained in the central bore 1130E. When the
junction member 1112E is forced in a generally distal direction
toward the seed body 1122E, as shown in FIG. 11K, the adjustable
tines 1110E are moved to an actuated position. As previously
described, each of the tines 1110E may be biased to extend in a
curled or hooked shape after being released from the central bore
1130E.
[0120] Referring now to FIGS. 11L-11N, alternate embodiments of the
seed may include adjustable tines that are not disposed in a
central bore of the seed body. For example, some embodiments of a
seed 1120L may include a plurality of adjustable tines 1110L that
are disposed in non-central bores 1130L that extend in a
longitudinal direction near the periphery of the seed body 1122L.
The adjustable tines 1110L of the seed 1120L may be forced from a
non-actuated position (e.g., FIG. 11L) to an actuated position
(e.g., FIG. 11M). In this embodiment, the seed 1120L includes a
helical tine 1105L that extends distally from the seed body 1122L
and includes three adjustable tines 1110L that each extend from a
common junction member 1112L. As shown in FIG. 11J, when the
adjustable tines 1110L are in a non-actuated position, the junction
member 1112L is offset from the seed body 1122L, and at least a
portion of the adjustable tines 1110L are constrained in the
associated peripheral bores 1130L. When the junction member 1112L
is forced in a generally distal direction toward the seed body
1122L, as shown in FIG. 11K, the adjustable tines 1110L are moved
to an actuated position. As previously described, each of the tines
1110L may be biased to extend in a curled or hook shape after being
released from its associated bore 1130L. The tines 1110L may also
extend from the sides of seed 1120L, such as through electrode
1135L, and could also operate to extend excitation signals from
electrode 1135L into the tissue.
[0121] Referring to FIG. 11N, this embodiment of the seed 1120L may
be directed to the targeted site of the heart wall using a delivery
catheter 615 and an elongate shaft 1160L. The elongated shaft 1160L
may include a detachment mechanism 1165L that engages/disengages
with the seed 1120L. In this embodiment, the detachment mechanism
1165L includes a threaded member that engages a complementary
threaded member of the seed's detachment mechanism 1145L. As
previously described, the seed 1120L may be rotated such that the
helical tine 1105L penetrates through the endocardium 606 and into
the myocardium 605. When the seed 1120L is properly positioned, a
force from an actuation rod 1170L may move the junction member
1112L in a distal direction toward the seed body 1122L. Such motion
causes the adjustable tines 1110L to extend from the distal ends of
the peripheral bores 1130L, thereby causing the adjustable tines
1110L and the helical tine 1105L to secure the seed 1120L to the
myocardium 605. After the adjustable tines 1110L are moved to the
actuated position, the elongate shaft 1160L may be rotated to
release the seed 1120L at the detachment mechanisms 1145L and
1165L, which permits the delivery catheter 615 and the elongated
shaft 1160L to be withdrawn from the implantation site.
[0122] As previously described, the seed body may be secured to the
heart tissue using tines, screws, barbs, hooks, or other fasteners.
FIGS. 11P-11U illustrate further examples of such attachment
mechanisms. Referring to FIG. 11P, some embodiments of a seed 1120P
may include a body screw 1106P and adjustable tines 1110P to secure
the seed 1120P to the myocardium 605. The body screw 1106P may
include threads that are wound around the seed body 1122P so that
rotation of the seed body 1122P causes that penetration through the
endocardium 606 and into the myocardium 605. The threads may be
interrupted and twisted in some circumstances to help ensure that
the seed 1120P does not back out of the tissue.
[0123] The adjustable tines 1110P may be actuated when a junction
member 1112P is moved in a distal direction toward the seed body
1122P. Referring to FIG. 11Q, some embodiments of a seed may
include a single adjustable tine that helps to secure the seed to
the myocardium 605. For example, the seed 1120Q may include a body
screw 1106Q and an adjustable tine 1110Q that is actuated by moving
a junction member 1112Q toward the seed body 1122Q.
[0124] The embodiment of FIGS. 11P-11Q may provide additional
benefits to advancing the seed 1120P into tissue. By providing a
more tapered end on the seed body 1122P and connecting the body
screw 1106Q to the seed body 1122P, the seed 1120P may create an
opening for the passage of the seed body 1122P more easily into the
tissue. In some cases where the body screw 1106Q is not used, the
distal portion of the helical tine can pass into the heart wall
tissue, but further progress may be blocked when the seed body
1122P abuts the tissue. Also, while the thread is shown in FIGS.
11P-11Q as being disposed tight to the seed body 1122P, it could
also be separated slightly from the seed body 1122P, particularly
around the front tapered portion of the seed body 1122P, and then
connected back to the seed body 1122P, for example, by a thin
webbed section that can itself cut into the tissue. While it is not
necessary for all embodiments that the seed body be placed into the
tissue, other appropriate arrangements may be used that allow the
seed body 1122 to enter into the tissue without significant
disruption to the physical structure of the tissue.
[0125] Referring to FIG. 11R, some embodiments of a seed may
include an adjustable barb that helps to secure the seed to the
myocardium 605. The adjustable barb may include biased extensions
that outwardly shift when no longer constrained in a bore. For
example, the seed 1120R may include a body screw 1106R that
transitions into a helical tine 1105R and an adjustable barb 1111R
that is actuated by moving a junction member 1112R toward the seed
body 1122R. Referring to FIG. 11S, some embodiments of a seed 1120S
may include a helical tine 1105S and an adjustable barb 1111S to
secure the seed 1120S to the myocardium 605. The adjustable barb
1111S may be actuated by moving a junction member 1112S toward the
seed body 1122S. Referring to FIG. 11T, some embodiments of a seed
may include one or more body barbs 1107T that help to secure the
seed to the myocardium 605. The body barbs 1107T may extend from
the seed body 1122T and acts as hooks that prevent the retraction
from the myocardium 605. For example, the seed 1120T may be fully
embedded in the myocardium 605 and include body barbs 1107T and
adjustable tines 1110T that can be actuated by moving a junction
member 1112T toward the seed body 1122T. Referring to FIG. 11U,
some embodiments of a seed 1120U may include body barbs 1107U and
an adjustable barb 1111U to secure the seed 1120U to the myocardium
605. The adjustable barb 1111U may be actuated by moving a junction
member 1112U toward the seed body 1122U.
[0126] Referring now to FIGS. 11V-11W, some embodiments of the
detachment mechanism between the elongate shaft and the seed may
include a locking member that is movable between an engaged
position (e.g., FIG. 11V) and a disengaged position (e.g., FIG.
11W). In such embodiments, the elongate shaft may have a
noncircular outer cross-section (such as a square or hexagonal
cross-sectional outer shape) to facilitate translation of
rotational motion to the seed body.
[0127] Referring to FIG. 11V, the seed 1120V may include a body
1122V and electrodes 1135V and 1136V, as described in previous
embodiments. Furthermore, the seed 1120V may include tines, screws,
barbs, hooks, or other fasteners (such as a helical tine 1105V,
adjustable tines 1110V that extend from a common junction member
1112V) as previously described. Also as previously described, the
seed 1120V may be directed by an elongated shaft 1160V through a
lumen 712 of a delivery catheter 615. The seed 1120V may include a
detachment mechanism 1145V having a cavity 1146V shaped to receive
at least a portion of a locking member 1166V. In the depicted
embodiment, the cavity 1146V may be curved to fit a spherically
shaped locking member 1166V like a small ball such that, when the
locking member 1166V is engaged with the cavity 1146V, the elongate
shaft 1160V is not retractable from the seed body 1122V.
[0128] Referring to FIG. 11W, when at least a portion of the seed
1120V is properly positioned in the myocardium 605, a force 1177V
may be applied from the actuation rod 1170V may be to move the
junction member 1112V toward the seed body 1122V. Such motion of
the junction member 1112V may cause the adjustable tines 1110V to
extend from the seed body 1122V, thereby securing the seed 1120V to
the myocardium 605. In addition, the motion of the actuation rod
1170V may cause the locking member to move to a disengaged
position. For example, the actuation rod 1170V may include a
depressed surface 1176V that is substantially aligned with the
locking member 1166V when the actuation rod 1170V forces the
junction member 1112V to actuate the tines 1110V. As such, the
locking member 1166V moves toward the depressed surface 1176V and
disengages with the cavity 1146V. This disengagement permits the
actuation rod 1170V, the elongate shaft 1160V, and the delivery
catheter 615 to be withdrawn from the seed implantation site while
at least a portion of the seed 1120V remains secured to the
myocardium 605.
[0129] Detachment mechanisms other than those discussed above may
also be used in appropriate situations. For example, multiple
spherically shaped locking members like that discussed above may be
attached along the length of a wire, such as by soldering. The wire
may be passed down an interior passage of multiple seeds that are
mounted end-to-end on the tip of a catheter. Each locking member
may be located so as to extend out of a central bore inside the
seeds to lock against a corresponding cavity on an internal surface
of a seed. In operation, and with locking member holding each seed
in place, the most distal seed may be driven into the tissue by
rotating the seeds. The wire may then be withdrawn proximally the
length of one seed, so that the locking member in the most distal
seed is pulled back to the second-most-distal seed, and the other
locking members move back one seed. Such a controlled withdrawal of
the wire may be accomplished, for example, using an indexed trigger
mechanism that is handled by the surgeon. The second seed--now the
most distal seed--may then be implanted, and the wire withdrawn
again. In such a manner, multiple seeds may be implanted from a
single introduction of the mechanism into a heart chamber.
[0130] In addition, the seeds may be provided with alternative
mechanisms for removal, such as for use when the primary attachment
mechanisms are damaged, occluded, or otherwise unavailable. For
example, several channels may be formed about the periphery of a
proximal, nonimplanted electrode. The channels may proceed from
shallow to deep so that, for example, a tool having
radially-arranged fingers with inward extensions may position those
extensions around the electrode. The fingers can then be
contracted, such as by a sleeve that is slid down around the
exterior of the fingers, and the extensions may be received into
the channels. The tool may then be rotated so that the extensions
move down into the deep portions of the channels and engage the
seed in rotation so that the seed may be removed from the
tissue.
[0131] FIG. 12 illustrates the possibility that seeds 1220 may be
placed parallel to the heart wall 605, in addition or in preference
to transverse placement. This may be particularly necessary where
the heart wall is thin, for example in the atria or in regions of
the ventricles that contain scar tissue. Placement parallel to the
wall is particularly required when the wall thickness is less than
the seed length. Note that the catheter 1212 may be curved near its
tip to facilitate parallel placement. Since the heart wall 605 is
moving during the cardiac cycle, it may be necessary to secure the
tip of the catheter 1212 to the heart tissue during seed placement.
This concept is illustrated in FIG. 13, showing a cork screw 1350
temporary securement of the catheter 1312 to the wall 605. Tines
that extend from the distal end of the catheter for penetration
into the heart wall to secure and stabilize the catheter tip during
seed delivery are also envisioned. The tines would be extended into
the heart wall before seed placement, and retracted from the heart
wall after seed placement.
[0132] FIGS. 14A and 14B show a seed embodiment in which a seed
pick-up coil 1460 also serves the function as a distal attachment,
extending into the epicardial space 1465. The seed includes a seed
body 1402, the distally extending coil 1460 and proximal tines
1465. The coil 1460 is wrapped down in a delivery tube 1470
provided by a catheter 1412, and expands to its full diameter after
being pushed into the epicardial space 1465. The seed is pushed
using a push rod, or wire, 1475 that operates to push the coil 1460
from the distal opening in the delivery tube 1470 and into the
epicardial space. The seed body 1402 and proximal tines remain
within the heart wall 605. The expanded coil 1460 has the advantage
of collecting more magnetic flux by virtue of its larger diameter,
leading to better coupling to the antenna, and a more efficient
pacing system. The seed in FIGS. 14A-B can have a reduced diameter
because it does not contain a relatively bulky coil. The seed body
1402 contains the capacitor and electronic components indicated in
the schematic of FIG. 4. Proximal tines 1465 are shown attached to
the seed for additional securement.
[0133] It is noted again, that it may be desirable to achieve
maximum spacing between the proximal and distal electrodes to
ensure conduction through the maximum volume of refractory tissue.
For example, it may be possible for the bullet shaped seed of FIG.
4 to become encapsulated in fibrous, non-refractory tissue. In this
case, the current density in tissue surrounding the fibrous capsule
may be too low to cause depolarization. A solution to this problem
is to use the furthest extremities of the seed as electrodes. For
example, tines 715, 720, 750 and 755 (see FIG. 7) may be plated
with a suitable conductive material to serve as electrodes that
extend into the epicardial space. Current passing between the
distal tines and the proximal seed electrode would then pass
through refractory tissues. As a further precaution, the proximal
tines 750 and 755 could be plated with a conductive material and
serve as an extension of proximal electrode 710. Current passing
between distal and proximal tines would encounter refractory
tissues with a high degree of probability. Similarly, the
epicardial coil 1460 of FIG. 14 may contain a central conducting
coil surrounded by an electrical insulator, which is in turn coated
with a conductive electrode material.
[0134] For completeness, shown in FIG. 15 is an alternative seed
coil embodiment in which three orthogonal coils are wound on a
single substrate. The substrate may be made from a permeable
material. Currents induced in each of the three coils would be
rectified, and passed to a single capacitor. In this embodiment,
the orientation of the seed relative to the transmit antenna is
immaterial. This is important because there is no coupling between
a coil having its axis parallel to the plane of the antenna, and it
may not always be possible to implant a seed with its axis
perpendicular to the plane of the antenna. The seed of FIG. 15
collects magnetic flux in each of three orthogonal directions, so
that maximum flux is collected independent of the orientation of
the incident magnetic field.
[0135] The electrical parameters in the seed circuit of FIG. 4, and
the geometry of the antenna 260 of FIG. 6 may be optimized by the
use of a computer model for the response of the seed to the
magnetic field generated by the antenna. The fundamental
requirement is that the energy stored on capacitor 405 of FIG. 4
after charging is complete be equal to the pacing threshold energy
for the tissue surrounding the seed. For example, conventional
pacemaker electrodes deliver on the order of four micro-Joules
(E.sub.0=4 .mu.J) of energy to pace the tissue each time the heart
beats. This number depends upon the tissue type, pulse shape, and
electrode geometry, but will be used here as an example. The total
energy required to pace N sites is then on the order of N times the
threshold energy E.sub.0. For example, if ten sites are paced using
ten seeds, then the total energy requirement will be on the order
of NE.sub.0=40 .mu.J for every heart beat. The energy that must be
supplied by the antenna 260 on each heartbeat is this minimum
pacing energy times the overall efficiency of coupling energy from
the antenna to seeds.
[0136] The energy delivered to each seed in a charging time, .tau.,
may be computed for a given set of seed circuit parameters and a
measured or computed magnetic field versus time at the site of the
seed in question. This is possible because the voltage induced in
coil 410 is known to be equal to the time rate of change of
magnetic flux linking the coil. The steps needed to compute the
energy stored on a given seed capacitor are:
[0137] For a given antenna shape, location and orientation, and
antenna current waveform, I(t): [0138] 1) Compute the magnetic flux
linking a seed coil 410 at a given location and a given orientation
relative to the antenna, residing in a tissue medium having
realistic frequency dependent values of electrical conductivity and
permittivity. [0139] 2) Compute voltage induced in the coil (and
modeled as a voltage in series with the coil 410) as the time rate
of change of the flux computed in step 1). [0140] 3) With the
switch 418 in position 1, use seed circuit equations to compute the
charge on capacitor 405 versus time, and therefore the energy
stored on the capacitor (equal to square of charge divided by two
times the capacitance of 405).
[0141] Generally speaking, the magnetic field falls off rapidly as
the separation between the seed and the antenna increases. While
this may not be true for very large antennas, the body dimensions
limit the practical dimensions of the antenna. The exact location
(and orientation if the seed does not have a tri-axial coil) of the
seed will determine the antenna current magnitude and ON-time
required to charge that seed. The seed that links the least
magnetic flux from the antenna will then determine these antenna
parameters, since all seeds must be capable of acquiring the
threshold energy for pacing. We may refer to this seed as the
"weakest link ", and it alone will be used to compute optimal
antenna current waveform and coupling efficiency.
[0142] The energy coupling efficiency is defined as the ratio of
the total energy delivered to the seed capacitors, NE.sub.0 ,
divided by the sum of all energy lost by the antenna during the
on-time. Antenna losses that may be included in simulations
include: [0143] Energy delivered to all seeds=NE.sub.0 [0144] Power
dissipated (as Ohmic heat) in seed circuit during charging [0145]
Power dissipated (as Ohmic heat) in antenna circuit during charging
[0146] Power dissipated (as Ohmic heat) by eddy currents induced in
conductive body tissues
[0147] The energy coupling efficiency is then given by NE.sub.0
divided by the sum of losses listed above over the duration of the
charging time. The Ohmic heat in the antenna circuit is primarily
due to I.sup.2R losses in the antenna itself, and hysteresis losses
in any magnetic materials that may be included in the antenna
design. This statement is also true for Ohmic heating in the seed
circuit. Once the parameters of the antenna current waveform needed
to charge the weakest link seed to the pacing threshold energy have
been determined, these losses may be computed. Once the antenna
current waveform parameters have been determined, the electric
field, E, generated at any point in the body may be computed. Then,
given a knowledge of the electrical conductivity of all body parts
affected by the antenna, the current density may be computed at any
point in the body as J=.sigma.E, where .SIGMA. is the electrical
conductivity at that point. The Ohmic heating due to eddy currents
is then found by integrating the power loss density
JE=.sigma.|E|.sup.2 over the volume of the patient's body. Since
both the magnetic field and the electric field produced by the
antenna waveform at any point in space may be derived from the
magnetic vector potential, the following further steps may be used
to compute coupling efficiency: [0148] 4) Compute the vector
potential, A, arising from a given current waveform in the seed
medium, using realistic tissue conductivity and permittivity.
[0149] 5) Compute the magnetic field at the site of the seeds as
B=curl(A) [0150] 6) From 5) determine antenna current waveform
parameters needed to charge the weakest link seed to the pacing
threshold energy [0151] 7) Compute antenna circuit losses for the
current waveform found in 6) [0152] 8) Compute the sum of all seed
circuit losses given a set of seed locations and orientations to
the field, and the field computed in 5) using 6) [0153] 9) Compute
the electric field at points in space as
E=-.differential.A/.differential.t [0154] 10) Integrate
.sigma.|E|.sup.2 over the patient's body using known or estimated
values for the electrical conductivity .sigma. at each point in
space to determine energy lost to absorption by body tissues [0155]
11) Compute efficiency as charging energy delivered to seeds
divided by the charging energy plus the losses computed in
7)-10)
[0156] Optimization of seed design, antenna design, and antenna
circuit waveform is performed by iterating steps 1)-11) to maximize
coupling efficiency. The lifetime of the transmitter battery is
readily computed from the energy coupling efficiency since on each
heart beat the antenna must supply the total pacing energy,
NE.sub.0 divided by the coupling efficiency. The total energy
contained in the battery is its volume times its energy density.
The total expected number of heartbeats that the system can pace is
then the total battery energy times the energy coupling efficiency
divided by the pacing energy per heartbeat, NE.sub.0. Making an
assumption about the average heart rate, say 72 beats per minute,
then yields the battery lifetime in minutes.
[0157] In one example calculation a seed contained a coil 3 mm long
by 2 mm diameter wound on a core with relative permeability equal
to ten. The capacitance was chosen to make the coil resonant at the
frequency of the applied magnetic field. A further constraint was
made by choosing the Q of the coil (resonant frequency divided by
the width of the resonance peak) equal to ten. This constraint of a
modest Q provides a margin for possible frequency dispersion by
conductive tissues, and a manufacturing margin. Given these
assumptions it was found that a magnetic field directed along the
axis of the coil must have a magnitude of about 0.001 Tesla (1 mT)
to provide the minimum pacing energy of 4 .mu.J. The antenna model
in this calculation was a five inch diameter circular loop of
copper having a total weight of 100 grams. The tissue model
employed was a combination of heart muscle and blood, having about
the same electrical conductivity. When the weakest link seed was
placed at a distance of three inches from the plane of the antenna,
the following was determined: The optimal energy coupling occurred
at a frequency of about 30,000 Hz (30 kHz), where efficiency peaked
at about 0.5%, and the lifetime of a 100 gram battery with 720
Joules/gram energy density was about 2 months.
[0158] The efficiency can be improved by improving magnetic
coupling between the seeds and the antenna. This may be
accomplished by using multiple antennas, for example one loop on
the ribs over the anterior side of the heart, and one loop on the
ribs over the posterior side of the heart. Two or more antenna
loops may insure that the weakest link seed is closer to a loop
than the three inches used in the example above. An alternative
location for an antenna loop may be a loop inserted into the right
ventricle of the heart, and attached to a controller placed at the
usual pectoral implant location. Such a loop would be located
closer to all seeds, particularly since the antenna is energized
during systole when the heart is contracted.
[0159] Battery lifetime can be extended indefinitely by employing a
rechargeable battery. The battery may receive energy for recharging
by inductive coupling to antenna 260. External antennae and
transmitters for recharging could be located under or around the
patient's bed or chair, or be integrated into special clothing. As
an alternative to a rechargeable battery, the antenna, transmitter,
and battery of FIG. 3 could be integrated into clothing or a
disposable patch worn by the patient. ECG signals needed to time
the seed pacing could be received via an inductive link from a
conventional pacemaker with right atrial and right ventricle leads.
In this case, elaborate antenna designs could be incorporated into
the special clothing. For example, the antenna could have a portion
that surrounds the chest at the latitude of the heart.
[0160] FIG. 16 shows a schematic diagram of an antenna 260 with the
charging current waveform being supplied by capacitive discharge
through the antenna 260, and capacitor recharge provided by a
battery 1605. The value chosen for the capacitor 1610 determines if
the current waveform has a single peak or whether the current rings
down in a damped sine waveform. Communications electronics 1615
sends pacing discharge signals to the seeds, but may also receive
ECG signals from the seeds or a conventional pacemaker. The charge
electronics 1620 receives energy via the antenna from an inductive
link to an external antenna, to recharge the battery. A control
circuit 1625 controls the operation of the recharge circuit 1620
and the communications electronics 1615.
[0161] It is also noted that alternative sources of power for the
seeds may be used. For example, the mechanical energy of the
beating heart is many orders of magnitude larger than the energy
required to pace the seeds. At the site of a seed, the heart muscle
thickens during systole and thins during diastole as the heart
beats. It is estimated that a one mm diameter transducer placed
across the heart muscle could generate 65 .mu.J of energy due to
the contraction of the heart, more than ten times the energy needed
to pace. A simple mechanical to electrical transducer having
nominal efficiency could provide the energy to pace a seed. Other
miniature local sources of energy have been suggested in recent
literature. These include: piezoelectric and electro-active polymer
materials that transduce mechanical to electrical energy;
bio-batteries that convert body heat and/or blood flow energy to
electrical energy; and tiny amounts of radioactive material that
emit short range alpha or beta particles that are readily
shielded.
[0162] In addition, the seed circuit of FIG. 4 can be simplified by
omission of the capacitor and voltage controlled switch. That is,
the seed circuit may consist simply of a coil connected across
electrodes in contact with tissue. In this case a magnetic field
pulse induces a voltage pulse in the seed coil, and the induced
voltage directly discharges into tissue. If all seeds are the same,
pacing of all seeds is simultaneous. However, the rise time of the
induced voltage can be adjusted by adjustment of the coil parameter
number of turns, core permeability, and adjustment of a resistor in
series with the coil. Thus, a collection of seeds having varying
rise times may be used to synchronize the firing sequence of the
seeds. The controller may sense a singe local ECG, for example the
atrial or right ventricle electrode of a special transmitting seed
or of a conventional pacemaker that transmits data to the
controller. A burst of current into the antenna would then fire all
seeds, with the precise time of firing determined by the electrical
properties of each implanted seed.
[0163] FIGS. 18A-18C show an end view, side view, and side view
with equivalent circuit for a simplified seed 1800 for delivering
stimulation to tissue, including myocardial tissue on the inside of
a heart chamber. As shown, the seed does not have separate energy
storage components such as a battery or a capacitor. It instead is
comprised of a ferrite core 1805 which may be in the form of a
cylinder approximately one mm in diameter and three mm long. At
each end of the core 1805 are ferrite caps 1810 which may be in the
form of circular disks about 1 mm thick and about 3 mm in diameter.
The caps 1810 may be attached to the end of the core 1805, may have
central holes through which the core 1805 is received, or may be
integrally formed with the core 1805. Ring electrodes 1815 may be
formed about the periphery of each cap. The ring electrodes 1815
may be formed of any appropriate materials such as platinum-iridium
alloy. The ring electrodes 1815 may be bonded to the caps 1810
using medical grade epoxy, cyanoacrelate, or the like. Other
arrangements for the electrodes and other components may also be
used, and the particular layout and shape of components that is
meant to be illustrative rather than limiting. Because the seed
does not have a distinct energy storage device such as a battery or
capacitor, it is referred to in this document as a direct
activation electrode assembly or device.
[0164] The seed 1800 may receive signals using a long loop of wire
1820 wrapped around the core. For example, 99.99% silver wire that
is 0.002 inches in diameter and is covered in a polyurethane nylon
insulation may be used. The wire 1820 may be wrapped around the
core 1805 in any appropriate manner and may comprise, for example,
about 900 turns of wire. In general, the voltage induced in the
coil is proportional to the number of turns of wire. Wire having a
smaller diameter yields more turns when the wire fills the empty
volume over the core (nominally 3 mm long gap with 3 mm outside
diameter and 1 mm inside diameter). However, smaller diameter wire
has a higher electrical resistance, and if the coil resistance
becomes comparable to the impedance of the tissue being paced, the
net energy delivered to the tissue will diminish. In general the
electrical resistance of the wire should not exceed a few hundred
Ohms. The measured electrical resistance of the 900 turns of wire
1820 is about 60 Ohms.
[0165] The seed 1800 may also be covered as appropriate to protect
the materials in the seed 1800 and to insulate them from the tissue
and fluids around the seed 1800. For example, a hermetic epoxy
layer 1830 may be applied to the ends of both caps 1810, and
another hermetic epoxy layer 1825 may be applied around the outside
of the coiled wire 1820. In general, the ring electrodes will not
be insulated, though they may otherwise be treated, so that they
can deliver sufficient energy to the tissue surrounding the seed
1800. The coil 1105E and/or one or more of tines 1110E, and/or the
seed distal curved face 1123E may be electrically connected to and
part of the distal electrode 1135E. Alternatively, one or more of
1105E, 1110E and 1123E may be used in place of ring 1135E as the
distal electrode.
[0166] In general, the seed 1800 should be small enough to be
delivered easily, such as through a 9 French delivery catheter.
Exemplary dimensions of such a seed are 5 mm long and 3 mm in
diameter. Also, the seed just described may be incorporated with
the delivery and anchoring mechanisms discussed earlier in this
document. Typical parameters for the seed 1800 would be a voltage
pulse amplitude greater than 0.5 volts (with 2 volts being
typical), and a pulse duration of approximately 0.4 msec. In
addition, to neutralize charge on the electrodes, the electrical
waveform that seed 1800 delivers to the tissue will generally have
the pacing pulse described above (with the distal electrode being
the cathode) followed by a smaller-amplitude, longer-duration pulse
of the opposite polarity so that the integral of the waveform over
time will be zero.
[0167] Advantageously, the described seed is extremely
uncomplicated and is thus capable of delivery one or more specific
benefits. First, the simple design allows the seed to take a very
small form factor. A small seed can be used with less tissue trauma
to a patient, and may also be implanted more easily and at more
locations using, for example, percutaneous tranluminal implantation
with catheters, as discussed above. This form factor can be reached
without extreme engineering for miniaturization, such as would be
required for a system using electrical storage devices in the
seed.
[0168] The simple design is also likely to provide excellent
reliability, as there are very few parts to the system, and very
little to wear out or otherwise fail. The simple design also
contributes to manufacturability, as the seed is fairly simple to
make, and thus should be lower in cost and also be manufactured
with fewer errors. In addition, the described antenna circuit is
small and simple, which may facilitate implantation, lower costs,
and improve manufacturability and reliability in similar ways.
[0169] The simple seeds also provide operational flexibility.
Specifically, the pacing waveform parameters may be adjusted at the
antenna circuit without a need to communicate with each of the
multiple implanted wireless electrodes. In addition, the seed can
provide extremely fast rise times (e.g., an "instant ON"
characteristic), which allows possible voltage limiters in the
seeds to give all electrodes the same pacing pulse amplitude with
nearly the same rise time.
[0170] The equivalent circuit attached to seed 1800 in FIG. 18C is
designed to represent the features of tissue around the seed 1800.
The equivalent circuit comprises two parallel impedances 1830,
1835, with impedance 1830 representing extra-celular conductive
fluid with a resistor, and impedance 1835 representing muscle cell
impedance by cell capacitance in series with a resistor
representing intra-cellular fluid. The equivalent circuit is useful
in testing candidate wireless electrode or seed designs to
determine which will provide the best treatment under particular
conditions. The equivalent circuit can also be used after the
design phase, during manufacture, to test seeds to ensure that they
are working properly. For example, manufactured seeds can be placed
in a magnetic field having a waveform substantially identical to
that used in the implanted systems, and their reaction may be
measured to ensure that they meet manufacturing requirements. In
this manner, the equivalent circuit may be particularly useful in
two phases of the process--design and manufacture.
[0171] The design of the seed can be expressed mathematically by
starting with an expression for the voltage induced around the
perimeter of an area element whose surface is perpendicular to a
time varying magnetic field: V.sub.ind=-A(dB/dT) (1) where
V.sub.ind=induced voltage in volts A=surface area in m.sup.2
B=applied magnetic field in Tesla
[0172] In Eq. (1), the magnetic field is assumed to be constant in
space over the area of the surface. The induced voltage is present
throughout space surrounding the source of the magnetic field. A
current will flow in a conductive element placed in the time
varying magnetic field. For example, the source of the magnetic
field may be a current pulse flowing in an antenna, as described
above. In a coil aligned with the external magnetic field, the
voltage of Eq. (1) is induced in each turn of the coil. If the coil
is wound on a magnetically permeable core material, the voltage is
further multiplied by the effective permeability of the core. If
the coil has multiple layers, the area of Eq. (1) is larger for
each successive layer.
[0173] Under these observations, the net voltage induced in a coil
wound on a permeable core is: V.sub.ind=-.beta.(dB/dt) (2) where
.beta.=.mu.N(.pi./12) (D.sub.i.sup.2+D.sub.iD.sub.o+D.sub.o.sup.2)
.mu.=effective permeability of the core (unitless) N=the total
number of windings on the coil D=inside diameter of the coil in
meters D.sub.o=outside diameter of the coil in meters
[0174] If the magnetic field is created by a pulse of current in
the antenna, then the time integral of the induced voltage in Eq.
(2) is zero, because the field itself is zero at both time zero and
after the pulse is delivered. Such a seed thus meets the standard,
discussed above, that the integral of the waveform over time is
zero.
[0175] Considering now the case of a magnetic field generated by a
circular loop antenna, the magnetic field at a distance, z, along
the axis from the center of a circular loop carrying a current, I,
is: B=(.mu..sub.oN.sub.aI/D)[1+(2z/D).sup.2].sup.-3/2=.gamma.I (3)
where .mu..sub.o=permeability of free space=4.pi..times.10-7
Weber/Amp-m N.sub.a=number of windings on the antenna D=antenna
diameter is meters z=distance along axis from antenna center in
meters .gamma.=(.mu..sub.oN.sub.a/D)[1+(2z/D).sup.2].sup.-3/2 in
Tesla/amp
[0176] The current, I, through the antenna may be made a pulse
whose time derivative yields an appropriate pacing waveform when
Eq. (3) is inserted into Eq. (2). A relatively simple circuit, like
that shown in FIG. 16 can produce an appropriate pulse. In that
figure, the capacitor 1610 may be charged to the voltage, V, of the
battery 1605. A microprocessor controller, such as control circuit
1625 may be configured to operate the switch near capacitor 1610
and may sense the p-wave in a patient's cardiac ECG The ECG may be
sensed, for example, near the site of the controller implant, or
via skin patch electrodes in the case of an external antenna.
Alternatively, an implanted sensing lead or wireless electrode may
transmit the ECG signal or p-wave trigger to he controller. When
the capacitor is switched across the circular loop antenna in FIG.
16, the current flowing in the antenna is given by:
I=(CVQ.sup.2/.tau.S)[(e.sup.-(1+S)(2)t/(2.tau.)-e.sup.-(1-S)t/(2.tau.)]
(4) where C=capacitance in farads V=voltage applied in volts
Q=quality factor (unitless)=(1/R)(L/C).sup.1/2 .tau.=L/R (time
constant) in seconds L=antenna inductance in Henries R=antenna and
capacitor resistance in Ohms S=(1-[2Q].sup.2).sup.1/2
[0177] Combining Eqs. (2)-(4) provides the voltage induced in the
wireless electrode coil:
V.sub.ind=.beta..gamma.(CVQ.sup.2/2.tau..sup.2S)[(1+S)e.sup.-(1+S)t/(2.ta-
u.)-(1-S)e.sup.-(1-S)t/(2.tau.)] (5)
[0178] By evaluating Eq. 5 numerically, one can determine that the
waveform is a damped sinusoid when Q>0.5, and is a pulse
waveform when Q<0.5. A pulse waveform is appropriate for pacing,
and by numerical evaluation of Eq. (5), the pulse has maximum
amplitude when Q=0.5. Thus, for this idealized model, antenna
components may be selected to achieve Q=0.5, so that Eqs. (4) and
(5) become (in the limit of Q.fwdarw.0.5 and S.fwdarw.0):
I=(CVt/4.tau..sup.2)e.sup.-t/2.tau. (6)
V.sub.ind=.beta..gamma.(CV/4.tau..sup.2)(1-t/2.tau.)e.sup.-t/2.tau.
(7)
[0179] The waveform of Eq. (7) has a positive pulse with a zero
crossing at t=2t, followed by a shallow negative wave that falls
exponentially with time. The wave form of Eq. (7) integrates to
zero, as is discussed above as being desirable. For a desired pulse
width of 0.4 msec, r is selected as 0.2 msec. Equation (7) is shown
plotted in FIG. 19, with a voltage at time zero taken as 0.23
volts. The solid line in the figure represents computed values,
while the triangles represent measure values using a seed like that
shown in FIGS. 18A and 18B. Specifically, the measured data was
taken with a seed electrode body 5 mm long comprising a coil wound
on a ferrite bobbin having core dimension of 1 mm and end flange
thickness of 1 mm on each end-the coil of wire being 3 mm long with
an inside diameter of 1 mm and an outside diameter of 3 mm, wound
on the ferrite bobbin with 900 turns of 0.002 inch insulated silver
wire. Using Eq. (2), these parameters produce a value of
.beta.=0.003 m.sup.2. The measurements were generated using an
antenna having a diameter of seven inches that was constructed from
four turns of AWG #8 copper wire.
[0180] The wireless electrode was placed at the center of the
circular antenna, where the parameters of Eq. (3) yield
.gamma.=2.8.times.10.sup.-5 Tesla/amp. The antenna circuit
capacitorhad C=0.02 Farads, and the applied voltage was V=15 volts.
With .tau.=0.2 msec, the voltage at time zero computed from Eq. (7)
and these parameter values is V.sub.ind=0.16 volts, compared to
V.sub.ind=0.23 volts in the computed plot of FIG. 19.
[0181] Further testing was conducted on seeds having end caps of
varying thickness, with the coil wound on a 1 mm ferrite core and
the gap filled with wound insulated silver wire. The seed with the
highest induced voltage had end caps 1 mm thick, with 3 mm of wound
wire between them, and a total diameter of 3 mm.
[0182] This seed was tested with and without the equivalent circuit
of FIG. 18C attached to the electrodes. FIG. 20 shows a plot of the
voltage induced in such a seed when it is placed at the center of
the seven inch circular loop antenna discussed above, with voltage
V=15 volts and conductance C=0.02 Farads. The figure indicates that
the wireless electrodes are not loaded down significantly by the
tissue impedance, and pacing voltages larger than one volt are
readily attained in the presence of tissue. The waveform of the
figure is also appropriate for cardiac pacing using a simple and
small wireless electrode and simple antenna circuit. A comparison
of FIG. 20 without the equivalent circuit and FIG. 19 shows that
the seed has an effective permeability of 1.8/0.18=10 (equal to the
ratio of peak induced voltages, since the seeds have the same
geometry and number of turns).
[0183] A passive voltage limiting element such as a Zener diode may
be added to the seed across the stimulation electrodes to control
the voltage pulse amplitude. For example, when multiple seeds are
located at multiple distances from the antenna, the magnitude of
the applied magnetic field will vary from seed to seed according to
Eq.(3). The voltage limiting element may help ensure that the pulse
amplitude is the same for all seeds and all antenna configurations
when the seeds are close enough to the antenna to generate the
limit voltage.
[0184] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the scope of the
invention. For example, although the disclosure discusses
embodiments in relation to cardiac tissue, the systems and methods
described herein are applicable to excitation of other cells,
tissues, and organs that may be stimulated to achieve some benefit
or result.
[0185] In some embodiments, the systems and methods described
herein may be used in certain neurological applications. For
example, the wireless electrode assemblies and the related systems
described herein may be employed to limit pain, control muscle
spasms, prevent seizures, treat neurohormonal disorders, and the
like.
[0186] In other embodiments, the leadless electrode assemblies may
be delivered through other conduits other than blood vessels. For
example, wireless electrode assemblies described herein may be
delivered through the esophagus to the stomach lining or other
tissue in the digestive tract. By using the electrode assemblies to
electrically stimulation of the stomach tissue or other tissue in
the digestive tract, the systems described herein may be used to
treat digestive disorders or control hunger sensations.
[0187] In certain embodiments, the wireless electrode assemblies
described herein may be deployed in the urogenital tract. In such
embodiments, organs tissue in the abdominal area may be accessed
percutaneously via catheters through the peritoneal space.
[0188] Also, the apparatuses, systems, and methods described herein
and related to leadless stimulation of tissue may be combined with
elements of other types of seeds and/or related apparatuses,
systems, and methods. Such elements may be other than those
described in this document, such as the seeds, also referred to as
microstimulators, and related elements of apparatuses, systems, and
methods described in co-pending application Ser. Nos. 10/607,963;
10/609,449; 11/034,190; 11/043,642; 10/607,962; 11/043,404;
10/609,452; 10/609,457; and 10/691,201, each of which is assigned
to Advanced Bionics Corporation, and each of which is incorporated
herein by reference in its entirety.
[0189] For example, the microstimulators described in these
applications may be employed as seeds (modified so as to provide an
appropriate excitation or stimulation signal), may be provided with
the delivery and attachment or anchoring features described herein,
and may be implanted using the devices and methods described
herein. Alternatively, the apparatuses, systems, and methods
related to seeds as described herein may be modified so as to
include at least one element of the apparatuses, systems, and
methods related to microstimulators described in these incorporated
applications. Such at least one element may relate to implantation
and/or explantation; fixation and/or anchoring or seeds and/or
microstimulators; power transfer and/or data communication between
seeds, microstimulators, and other implanted or external power
transfer and/or data communications devices; methods of
manufacture; electronic circuitry; mechanical packaging of
hermetically-sealed seeds and/or microstimulators; materials; and
all other elements of apparatuses, systems, and methods described
in these incorporated applications. cm What is claimed is:
* * * * *